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/service_accounts.go

@@ -41,10 +41,14 @@ func callerIsAdmin(ctx context.Context) bool {
 	return ok && u.IsAdmin
 }
 
-// callerOwnsTeam reports whether the caller is a human owner of teamID. Built
-// on callerRole/ctxTeams like requireTeamOwner, but without writing a
-// response: callers here need to combine it with other ways of being
-// allowed, not stop at the first no.
+// callerOwnsTeam reports whether the caller is owner-equivalent for teamID:
+// a human owner, or that team's own team-scoped service account (its single
+// synthetic membership, serveAsServiceAccount — ratified in
+// SERVICE-ACCOUNTS.md as intentional, not an accident: a team-scoped
+// credential is that team's owner's reach, full stop, membership and
+// invites included). Built on callerRole like requireTeamOwner, but without
+// writing a response: callers here need to combine it with other ways of
+// being allowed, not stop at the first no.
 func callerOwnsTeam(ctx context.Context, teamID int64) bool {
 	role, ok := callerRole(ctx, teamID)
 	return ok && role == models.RoleOwner
@@ -173,9 +177,24 @@ func fetchServiceAccount(ctx context.Context, db *sql.DB, id int64) (models.Serv
 
 // callerMayManageServiceAccount reports whether the caller may mint or revoke
 // a key on sa: a system administrator, that team-scoped account's own human
-// owner, or the account rotating its own credential — which is not a
-// privilege escalation, the same reasoning requireSelfOrAdmin already rests
-// on for a user's own API keys.
+// owner, the account rotating its own credential (not a privilege
+// escalation, the same reasoning requireSelfOrAdmin already rests on for a
+// user's own API keys) — or, new, an instance-scoped service account
+// managing any team-scoped account.
+//
+// That last branch closes terdut-operator#3: handleCreateServiceAccount
+// already lets an instance-scoped caller *create* a team-scoped account for
+// any team (the branch below it, isInstanceServiceAccount(ctx)) — this
+// account didn't have an equivalent reach to *adopt or rotate* one it
+// didn't just create in the same call, which is exactly the recovery path
+// terdut-operator's own documented crash-window handling depends on
+// (DESIGN.md §5's general adopt-on-conflict rule): a reconcile that creates
+// the account successfully but crashes before persisting its credential
+// locally retries into a 409, and without this branch the only available
+// recovery — minting a fresh key on the now-existing account — 403'd
+// forever, with no way out. Granting it here is not a new power: it
+// mirrors the create-time reach this scope already has, just extended to
+// the retry path DESIGN.md's own crash-window reasoning requires.
 func callerMayManageServiceAccount(ctx context.Context, sa models.ServiceAccount) bool {
 	if callerIsAdmin(ctx) {
 		return true
@@ -183,7 +202,11 @@ func callerMayManageServiceAccount(ctx context.Context, sa models.ServiceAccount
 	if sa.TeamID != nil && callerOwnsTeam(ctx, *sa.TeamID) {
 		return true
 	}
-	if self, ok := serviceAccountFromContext(ctx); ok && self.id == sa.ID {
+	caller, _ := callerFromContext(ctx)
+	if id, ok := caller.ServiceAccountID(); ok && id == sa.ID {
+		return true
+	}
+	if sa.TeamID != nil && caller.IsInstanceServiceAccount() {
 		return true
 	}
 	return false