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/lib/exiftool.ts

@@ -51,7 +51,7 @@ export async function inspectMetadata(buffer: Buffer, filename: string): Promise
   try {
     await writeFile(tempPath, buffer);
     const { stdout } = await execFileAsync(bin, ["-json", "-G", "-struct", "-n", tempPath], {
-      timeout: 30_000,
+      timeout: 60_000,
       maxBuffer: 10 * 1024 * 1024,
     });
 
@@ -126,7 +126,7 @@ export async function writeMetadata(
   try {
     await writeFile(tempPath, buffer);
     await execFileAsync(bin, ["-overwrite_original", ...tags, tempPath], {
-      timeout: 30_000,
+      timeout: 60_000,
       maxBuffer: 10 * 1024 * 1024,
     });
     return await readFile(tempPath);