internal/api: unify human/service-account authz into one Caller type

ctxUser/ctxTeams (human) and ctxServiceAccount (+ a synthetic ctxTeams
entry, service account) used to be two parallel, un-unified context
representations -- every authz predicate had to remember which one(s) it
needed, and every place that forgot either wrongly 403'd a service account
(terdut-server#23, terdut-operator#3), crashed on an unchecked zero-value
user id, or silently no-op'd. New internal/api/caller.go collapses both
into one Caller, stored under one ctxCaller key by serveAs/serveAsServiceAccount;
every existing predicate (userFromContext, callerTeamIDs, callerRole,
callerIsAdmin, isInstanceServiceAccount, AdminOnly, requireSelfOrAdmin,
requireTeamOwner, OperatorModeBlock) now reads through it, with identical
behavior for every untouched call site (alerts.go, incidents.go,
schedule.go, stats.go, etc.) -- confirmed by the full existing suite
passing unchanged.

Four real fixes land alongside the refactor, not just the restructuring:

1. callerMayManageServiceAccount gains the one load-bearing branch this
   exists for: an instance-scoped service account may now manage (mint or
   revoke a key on) any team-scoped account, not only a human admin, that
   team's human owner, or the account itself. handleCreateServiceAccount
   already let an instance-scoped caller *create* a team-scoped account for
   any team; adopting or rotating one it didn't just create in the same
   call -- terdut-operator's own documented crash-window recovery -- had no
   equivalent permission and 403'd forever. Closes terdut-operator#3.

2. handleCreateInvite wrote a service-account caller's zero-value user id
   straight into invites.created_by (nullable, but never passed as nil),
   which foreign-key-violates against users(id) -- a 500, not success, for
   any team-scoped service account minting an invite. Fixed the same way
   handleCreateServiceAccount already handles the analogous case. Found
   live while verifying this change, not filed separately since it's fixed
   in the same place it was found.

3. handleMe and handleTestNotification 500'd for a service-account caller
   (fetchUser/the ntfy_topic lookup against a zero-value user id that
   matches no row); handleDismissOnboarding silently no-op'd (UPDATE ...
   WHERE id = 0). All three now call Caller.AsHuman() and return an
   explicit 403 ("this endpoint is for human accounts only").

4. Ratifies, rather than further narrows, two capabilities a team-scoped
   service account already had by construction and this document's own
   text once called "a gap acknowledged rather than closed": owner-equivalent
   reach over membership/invites, and minting another service account for
   its own team. terdut-operator's new TerdutTeam invite-minting feature is
   about to depend on the first one, so this makes it documented, tested,
   intentional behavior instead of an accident nobody was supposed to rely
   on.

AdminOnly/requireSelfOrAdmin are unchanged in effect: still human-only,
forever, for every scope of service account -- confirmed by
TestAdminOnly_RefusesEveryServiceAccountScope. terdut-server#23's named
routes (POST /api/users, PUT /api/admin/settings) were never the right
thing to widen; its real fix is the terdut-operator invite feature,
recorded in SERVICE-ACCOUNTS.md's "What this unblocks" and closing that
issue once it ships.

SERVICE-ACCOUNTS.md amended in place (not a new file, its own established
convention) to describe the as-built Caller model, correct its own
aspirational claim about AdminOnly that TEAM-LOOKUP.md had already flagged
as not matching shipped code, and record all of the above.

internal/api/middleware.go

@@ -16,10 +16,13 @@ import (
 type contextKey string
 
 const (
-	ctxUser           contextKey = "user"
-	ctxSession        contextKey = "session"
-	ctxTeams          contextKey = "teams"
-	ctxServiceAccount contextKey = "service_account"
+	// ctxCaller holds the one Caller (see caller.go) every authorization
+	// predicate in this package reads from — a human and a service account
+	// used to be two parallel, un-unified context keys (ctxUser/ctxTeams vs.
+	// ctxServiceAccount); this is why that was a mistake, not a smaller
+	// version of the same idea.
+	ctxCaller  contextKey = "caller"
+	ctxSession contextKey = "session"
 )
 
 // AuthMiddleware accepts either of the two credentials the server issues: an
@@ -179,8 +182,7 @@ func serveAs(w http.ResponseWriter, r *http.Request, next http.Handler, db *sql.
 		return
 	}
 
-	ctx := context.WithValue(r.Context(), ctxTeams, teams)
-	ctx = context.WithValue(ctx, ctxUser, u)
+	ctx := context.WithValue(r.Context(), ctxCaller, Caller{user: &u, memberships: teams})
 	if sessionID != 0 {
 		ctx = context.WithValue(ctx, ctxSession, sessionID)
 	}
@@ -192,9 +194,13 @@ func hashToken(token string) string {
 	return hex.EncodeToString(h[:])
 }
 
+// userFromContext is a thin compatibility wrapper over Caller.AsHuman(), so
+// every call site written before the Caller abstraction (alerts.go,
+// incidents.go, schedule.go, stats.go, and more) needs no change and keeps
+// its exact existing behavior.
 func userFromContext(ctx context.Context) (models.User, bool) {
-	u, ok := ctx.Value(ctxUser).(models.User)
-	return u, ok
+	c, _ := callerFromContext(ctx)
+	return c.AsHuman()
 }
 
 // serviceAccountPrincipal is a service account as resolved from its key:
@@ -244,26 +250,21 @@ func serviceAccountFor(ctx context.Context, db *sql.DB, token string) (serviceAc
 // key is only ever set by the client that holds it, never attached by a
 // browser to a request another site makes.
 func serveAsServiceAccount(w http.ResponseWriter, r *http.Request, next http.Handler, sa serviceAccountPrincipal) {
-	ctx := r.Context()
+	var memberships []membership
 	if sa.scope == models.ServiceAccountScopeTeam {
-		ctx = context.WithValue(ctx, ctxTeams, []membership{{teamID: sa.teamID, role: models.RoleOwner}})
+		memberships = []membership{{teamID: sa.teamID, role: models.RoleOwner}}
 	}
-	ctx = context.WithValue(ctx, ctxServiceAccount, sa)
+	ctx := context.WithValue(r.Context(), ctxCaller, Caller{sa: &sa, memberships: memberships})
 	next.ServeHTTP(w, r.WithContext(ctx))
 }
 
-func serviceAccountFromContext(ctx context.Context) (serviceAccountPrincipal, bool) {
-	sa, ok := ctx.Value(ctxServiceAccount).(serviceAccountPrincipal)
-	return sa, ok
-}
-
-// isInstanceServiceAccount reports whether the caller is an instance-scoped
-// service account — the one identity allowed to create a team and mint a
-// team-scoped account against any of them, the two things system
-// administration can already do that this extends to automation.
+// isInstanceServiceAccount is a thin compatibility wrapper over
+// Caller.IsInstanceServiceAccount(), for call sites outside this package's
+// core predicates (handleCreateTeam, handleCreateServiceAccount) that
+// needed this exact, narrow check before the Caller abstraction existed.
 func isInstanceServiceAccount(ctx context.Context) bool {
-	sa, ok := serviceAccountFromContext(ctx)
-	return ok && sa.scope == models.ServiceAccountScopeInstance
+	c, _ := callerFromContext(ctx)
+	return c.IsInstanceServiceAccount()
 }
 
 // operatorReason marks a write that operator mode refused as such, distinct
@@ -291,7 +292,8 @@ func OperatorModeBlock(cfg config.Config) func(http.Handler) http.Handler {
 			return next
 		}
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-			if _, ok := serviceAccountFromContext(r.Context()); ok {
+			caller, _ := callerFromContext(r.Context())
+			if _, ok := caller.ServiceAccountID(); ok {
 				next.ServeHTTP(w, r)
 				return
 			}
@@ -333,24 +335,15 @@ func callerMemberships(ctx context.Context, db *sql.DB, userID int64) ([]members
 // administration is about accounts, not about reading other people's incidents,
 // and an admin who needs to see a team's queue can add themselves to it.
 func callerTeamIDs(ctx context.Context) []int64 {
-	ms, _ := ctx.Value(ctxTeams).([]membership)
-	ids := make([]int64, 0, len(ms))
-	for _, m := range ms {
-		ids = append(ids, m.teamID)
-	}
-	return ids
+	c, _ := callerFromContext(ctx)
+	return c.TeamIDs()
 }
 
 // callerRole reports the caller's role in one team, and whether they are in it
 // at all.
 func callerRole(ctx context.Context, teamID int64) (string, bool) {
-	ms, _ := ctx.Value(ctxTeams).([]membership)
-	for _, m := range ms {
-		if m.teamID == teamID {
-			return m.role, true
-		}
-	}
-	return "", false
+	c, _ := callerFromContext(ctx)
+	return c.Role(teamID)
 }
 
 // requireTeamMember answers the request and reports false unless the caller