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).

frontend/package.json

@@ -6,6 +6,7 @@
     "dev": "next dev --webpack",
     "dev:turbo": "next dev",
     "build": "next build --webpack",
+    "build:local": "cross-env NEXT_OUTPUT=default next build --webpack",
     "build:turbo": "next build",
     "start": "next start",
     "lint": "eslint",
@@ -33,7 +34,7 @@
     "agentation-mcp": "^1.2.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
-    "framer-motion": "^12.23.24",
+    "framer-motion": "11.18.2",
     "lucide-react": "^0.555.0",
     "next": "16.0.6",
     "next-intl": "^4.8.3",
@@ -54,6 +55,7 @@
     "@types/react-dom": "^19",
     "@vitejs/plugin-react": "^5.1.4",
     "agentation": "^3.0.2",
+    "cross-env": "^10.1.0",
     "eslint": "^9",
     "eslint-config-next": "16.0.6",
     "jsdom": "^28.1.0",