Address production-readiness review: clearer errors, safer votes, and ops hardening.

Distinguish auth/lookup failures, make votes idempotent on visible questions, bound shutdown, page admin users, LRU throttle, trusted-proxy CIDRs, avatar cleanup, versioned migrations, and session cleanup logging.

internal/store/postgres.go

@@ -63,13 +63,9 @@ func OpenPostgres(databaseURL, schema string) (*sql.DB, *SessionStore, error) {
 		_ = db.Close()
 		return nil, nil, fmt.Errorf("postgres ping: %w", err)
 	}
-	if err := applySchema(db, schema); err != nil {
+	if err := applyMigrations(db, schema); err != nil {
 		_ = db.Close()
-		return nil, nil, fmt.Errorf("apply schema: %w", err)
-	}
-	if err := migrateUserProfileColumns(db); err != nil {
-		_ = db.Close()
-		return nil, nil, fmt.Errorf("migrate profile columns: %w", err)
+		return nil, nil, fmt.Errorf("migrate: %w", err)
 	}
 	sessions := NewSessionStore(db, 5*time.Minute)
 	return db, sessions, nil