Add photo content-hash dedup and local↔S3 library sync.

Uploads skip per-gallery duplicates, checksums can be backfilled, and STORAGE_BACKEND plus sync tooling make switching storage backends safe.

photo-api/internal/store/db.go

@@ -54,6 +54,19 @@ func Open(cfg config.Config) (*DB, error) {
 	}
 }
 
+// IsUniqueViolation reports whether err is a duplicate-key error. The two
+// drivers wrap it in unrelated types (pgconn.PgError vs sqlite.Error), and
+// neither is worth importing here just for one check, so this matches on the
+// message: pgx renders "(SQLSTATE 23505)", modernc/sqlite "UNIQUE constraint
+// failed: ...".
+func IsUniqueViolation(err error) bool {
+	if err == nil {
+		return false
+	}
+	msg := err.Error()
+	return strings.Contains(msg, "SQLSTATE 23505") || strings.Contains(msg, "UNIQUE constraint failed")
+}
+
 // Rebind converts ?-style placeholders to $n for Postgres. Queries in this
 // package never contain literal question marks in strings.
 func (db *DB) Rebind(query string) string {