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/timeout.ts

@@ -0,0 +1,26 @@
+import { env } from "../config.js";
+
+type ToolCategory = "sharp" | "ai_cpu" | "ai_gpu" | "external" | "python";
+
+const TIMEOUT_RATES: Record<ToolCategory, number> = {
+  sharp: 2,
+  ai_cpu: 30,
+  ai_gpu: 5,
+  external: 10,
+  python: 15,
+};
+
+export function computeTimeout(megapixels: number, category: ToolCategory, fileCount = 1): number {
+  if (env.PROCESSING_TIMEOUT_S > 0) {
+    return env.PROCESSING_TIMEOUT_S * 1000;
+  }
+  const perFile = Math.max(60_000, megapixels * TIMEOUT_RATES[category] * 1000);
+  return perFile * fileCount;
+}
+
+export function computeExternalToolTimeout(megapixels: number): number {
+  if (env.PROCESSING_TIMEOUT_S > 0) {
+    return env.PROCESSING_TIMEOUT_S * 1000;
+  }
+  return Math.max(60_000, megapixels * TIMEOUT_RATES.external * 1000);
+}