chore: remove dead code, add test infrastructure, update docs

- Delete 3 dead files: use-batch-processor.ts, use-i18n.ts, smart-crop.ts (AI package)
- Remove dead getJobProgress function and unused runPythonScript wrapper
- Remove 6 unused imports across API and web apps
- Remove unused shared types (ImageFormat, AppConfig, ApiError, HealthResponse, JobProgress)
  and constants (SUPPORTED_INPUT_FORMATS/OUTPUT_FORMATS, DEFAULT_OUTPUT_FORMAT)
- Remove unused store method (setOriginalBlobUrl) and clean AI package re-exports
- Add test infrastructure: vitest config, unit/integration/e2e tests, fixtures, screenshots
- Add Docker test infrastructure: Dockerfile.test, docker-compose.test.yml
- Add download_models.py for pre-baking AI model weights in Docker
- Add filename sanitization utility (apps/api/src/lib/filename.ts)
- Update .gitignore to exclude coverage/, *.tsbuildinfo, .superpowers/, test artifacts
- Update .dockerignore to exclude test/coverage/IDE artifacts from builds
- Update docs: remove smart crop from AI docs (uses Sharp directly), update bridge docs

apps/api/src/routes/tools/collage.ts

@@ -5,6 +5,7 @@ import { randomUUID } from "node:crypto";
 import { writeFile } from "node:fs/promises";
 import { join, basename } from "node:path";
 import { createWorkspace } from "../../lib/workspace.js";
+import { validateImageBuffer } from "../../lib/file-validation.js";
 
 const settingsSchema = z.object({
   layout: z.enum(["2x2", "3x3", "1x3", "2x1", "3x1", "1x2"]).default("2x2"),
@@ -54,6 +55,14 @@ export function registerCollage(app: FastifyInstance) {
         return reply.status(400).send({ error: "No images provided" });
       }
 
+      // Validate all files
+      for (const file of files) {
+        const validation = await validateImageBuffer(file.buffer);
+        if (!validation.valid) {
+          return reply.status(400).send({ error: `Invalid file "${file.filename}": ${validation.reason}` });
+        }
+      }
+
       let settings: z.infer<typeof settingsSchema>;
       try {
         const parsed = settingsRaw ? JSON.parse(settingsRaw) : {};