perf+security: fix build, secure downloads, dedupe translations, refactor i18n

Frontend:
- Fix Framer Motion / motion-dom build error by pinning framer-motion to
  11.18.2 (compatible with React 19 and Next.js 16).
- Add cross-env and build:local script to bypass standalone symlink errors
  on Windows without Developer Mode.
- Allow NEXT_OUTPUT=default to disable standalone output for local builds.
- Refactor i18n: split 14,177-line src/lib/i18n.tsx into per-locale,
  per-namespace JSON files under src/lib/i18n/messages/.
- Load English synchronously; other locales loaded on demand via dynamic
  imports (reduces initial bundle, improves maintainability).
- Remove unused next-intl message files src/messages/en.json and fr.json.

Backend:
- Remove insecure legacy /api/v1/download/{filename} and /api/v1/cleanup/{filename}
  endpoints. The job-based /api/v1/download/{job_id} already enforces ownership.
- Deduplicate texts in TranslationService.translate_batch before sending them
  to the provider, reducing API calls for repeated strings.
- Pin httpx to <0.28 to fix TestClient incompatibility with starlette 0.35.1.
- Add pytest-cov and ruff dev dependencies/config.

DevOps:
- Remove hardcoded Grafana password from docker-compose.yml and
  docker-compose.monitoring.yml; use GRAFANA_PASSWORD env var.
- Change default TRANSLATION_SERVICE from ollama to google in
  docker-compose.yml (Ollama is an optional profile).
- Add GRAFANA_PASSWORD to .env.example.
- Add .coverage and frontend/pnpm-workspace.yaml to .gitignore.

Tests:
- Update API versioning tests for removed legacy endpoints.
- Add tests/test_translation_service.py for deduplication behavior.

Verified:
- pnpm run build:local passes.
- uv run pytest tests/test_providers/* tests/test_translation_service.py
  tests/test_story_3_5_api_versioning.py tests/test_download_endpoint.py
  tests/test_translators/test_excel_translator.py: provider/translator tests
  pass; one pre-existing French error-message test still fails (message is
  returned in English, unrelated to this change).

scripts/generate_i18n_index.py

@@ -0,0 +1,39 @@
+"""
+Generate per-locale index.ts files that merge all namespace JSON files.
+
+Creates:
+  frontend/src/lib/i18n/messages/<locale>/index.ts
+"""
+
+import json
+from pathlib import Path
+from collections import defaultdict
+
+ROOT = Path(__file__).parent.parent
+MESSAGES_DIR = ROOT / "frontend" / "src" / "lib" / "i18n" / "messages"
+
+manifest = json.loads((MESSAGES_DIR / "index.json").read_text(encoding="utf-8"))
+
+for locale in manifest["locales"]:
+    locale_dir = MESSAGES_DIR / locale
+    namespaces = sorted(p.stem for p in locale_dir.glob("*.json"))
+
+    imports = "\n".join(f'import {ns} from "./{ns}.json";'
+                        for ns in namespaces)
+    export_obj = "\n  ".join(f"...{ns}," for ns in namespaces)
+
+    index_ts = f'''// Auto-generated by scripts/generate_i18n_index.py
+// Merges all namespace JSON files for locale "{locale}".
+
+{imports}
+
+const messages: Record<string, string> = {{
+  {export_obj}
+}};
+
+export default messages;
+'''
+
+    (locale_dir / "index.ts").write_text(index_ts, encoding="utf-8")
+
+print(f"Generated index.ts for {len(manifest['locales'])} locales")