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/signup.go

@@ -359,7 +359,19 @@ func handleCreateInvite(db *sql.DB, publicURL string) http.HandlerFunc {
 			respond(w, http.StatusInternalServerError, errResp("internal error"))
 			return
 		}
-		caller, _ := userFromContext(r.Context())
+		// created_by is nullable (ON DELETE SET NULL) for exactly this
+		// reason: the caller minting an invite is not always a human with a
+		// real users row. A team-scoped service account is owner-equivalent
+		// here (requireTeamOwner above already let it through), and this
+		// must leave created_by NULL for one the same way
+		// handleCreateServiceAccount already does for the analogous case —
+		// an unchecked zero value would violate the users(id) foreign key
+		// instead of recording "nobody" cleanly.
+		var createdBy *int64
+		if u, ok := userFromContext(r.Context()); ok {
+			id := u.ID
+			createdBy = &id
+		}
 		expires := time.Now().Add(inviteTTL)
 
 		var out inviteJSON
@@ -368,7 +380,7 @@ func handleCreateInvite(db *sql.DB, publicURL string) http.HandlerFunc {
 			INSERT INTO invites (token_hash, team_id, role, created_by, expires_at, max_uses)
 			VALUES ($1, $2, $3, $4, $5, $6)
 			RETURNING id, team_id, role, created_at, expires_at, max_uses, uses`,
-			hash, teamID, req.Role, caller.ID, expires.Unix(), req.MaxUses).
+			hash, teamID, req.Role, createdBy, expires.Unix(), req.MaxUses).
 			Scan(&out.ID, &out.TeamID, &out.Role, &created, &expiresAt, &out.MaxUses, &out.Uses); err != nil {
 			respond(w, http.StatusInternalServerError, errResp("internal error"))
 			return
@@ -429,7 +441,11 @@ func handleTestNotification(cfg NotifyConfig, db *sql.DB) http.HandlerFunc {
 				errResp("this server has no ntfy configured, so it can send nothing"))
 			return
 		}
-		caller, _ := userFromContext(r.Context())
+		caller, ok := userFromContext(r.Context())
+		if !ok {
+			respond(w, http.StatusForbidden, errResp("this endpoint is for human accounts only"))
+			return
+		}
 
 		var topic *string
 		if err := db.QueryRowContext(r.Context(),
@@ -470,7 +486,11 @@ func handleDismissOnboarding(db *sql.DB) http.HandlerFunc {
 			respond(w, http.StatusBadRequest, errResp("dismissed is required"))
 			return
 		}
-		caller, _ := userFromContext(r.Context())
+		caller, ok := userFromContext(r.Context())
+		if !ok {
+			respond(w, http.StatusForbidden, errResp("this endpoint is for human accounts only"))
+			return
+		}
 
 		var err error
 		if *req.Dismissed {