feat: dynamic timeouts — scale with image size, respect PROCESSING_TIMEOUT_S

Create timeout.ts utility for dynamic timeout computation.
Replace hardcoded timeouts across the stack:
- tool-factory worker: 30s → dynamic based on megapixels
- Python bridge default: 300s → 600s (or env override)
- background-removal: fixed → dynamic based on image size
- OCR: fixed 600s → dynamic based on image size
- seam-carving: 120s → dynamic based on image size
- ExifTool: 30s → 60s
- HEIC converter: 30s → 120s
- SQLite busy_timeout: 5s → 10s

apps/api/src/db/index.ts

@@ -13,7 +13,7 @@ const sqlite: DatabaseType = new Database(env.DB_PATH);
 // Critical SQLite pragmas for reliability.
 // busy_timeout must be set first so journal_mode = WAL can retry
 // if another connection holds the lock (e.g. parallel test files).
-sqlite.pragma("busy_timeout = 5000");
+sqlite.pragma("busy_timeout = 10000");
 sqlite.pragma("journal_mode = WAL");
 sqlite.pragma("synchronous = NORMAL");
 sqlite.pragma("foreign_keys = ON");