refactor(server): audit.WriteOrLog wrapper; migrate 13 sites (T5)

Add audit.WriteOrLog: a one-line wrapper around Write that logs at
Warn and swallows the error, matching the package contract that
audit failures must not break user-facing operations.

Migrate the 13 call sites across 7 files in internal/api/ from the
3-line "if err != nil { logger.Warn(...) }" shape to a single call.
audit.Write stays exported for tests + any future caller that
needs strict semantics.

Adds three tests: success (no log), failure-via-closed-pool (Warn
record with action+err keys), and nil-logger (no panic). Tests
skip when MINSTREL_TEST_DATABASE_URL is unset, matching the
existing harness convention.

internal/api/admin_invites.go

@@ -93,10 +93,8 @@ func (h *handlers) handleCreateInvite(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if err := audit.Write(r.Context(), h.pool, user.ID, pgtype.UUID{}, audit.ActionInviteCreate,
-		map[string]any{"token": token}); err != nil {
-		h.logger.Warn("admin: invite-create audit failed", "err", err)
-	}
+	audit.WriteOrLog(r.Context(), h.pool, h.logger, user.ID, pgtype.UUID{}, audit.ActionInviteCreate,
+		map[string]any{"token": token})
 
 	writeJSON(w, http.StatusOK, inviteFromRow(row))
 }
@@ -127,10 +125,8 @@ func (h *handlers) handleDeleteInvite(w http.ResponseWriter, r *http.Request) {
 		writeErrWithLog(w, h.logger, "admin: delete invite failed", apierror.Internal(err))
 		return
 	}
-	if err := audit.Write(r.Context(), h.pool, user.ID, pgtype.UUID{}, audit.ActionInviteRevoke,
-		map[string]any{"token": token}); err != nil {
-		h.logger.Warn("admin: invite-revoke audit failed", "err", err)
-	}
+	audit.WriteOrLog(r.Context(), h.pool, h.logger, user.ID, pgtype.UUID{}, audit.ActionInviteRevoke,
+		map[string]any{"token": token})
 	w.WriteHeader(http.StatusNoContent)
 }