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).

routes/legacy_routes.py

@@ -21,26 +21,6 @@ logger = logging.getLogger(__name__)
 router = APIRouter(prefix="/api/v1", tags=["Legacy"])
 
 
-def _safe_output_path(filename: str):
-    """
-    Resolve filename to a path under config.OUTPUT_DIR. Prevents path traversal.
-    Returns (Path, True) if valid, (None, False) if invalid.
-    """
-    if not filename or ".." in filename or "/" in filename or "\\" in filename:
-        return None, False
-    safe_name = Path(filename).name
-    if not safe_name.strip():
-        return None, False
-    base = config.OUTPUT_DIR.resolve()
-    try:
-        resolved = (config.OUTPUT_DIR / safe_name).resolve()
-        if not resolved.is_relative_to(base):
-            return None, False
-        return resolved, True
-    except (ValueError, OSError):
-        return None, False
-
-
 def _resolve_model(
     cfg_model: Optional[str],
     model_env: str,
@@ -340,41 +320,6 @@ async def translate_batch_documents(
     }
 
 
-@router.get("/download/{filename}")
-async def download_file(filename: str):
-    """Download a translated file by filename. Filename is sanitized to prevent path traversal."""
-    file_path, ok = _safe_output_path(filename)
-    if not ok or file_path is None:
-        raise HTTPException(status_code=400, detail="Invalid filename")
-    if not file_path.exists():
-        raise HTTPException(status_code=404, detail="File not found")
-    return FileResponse(
-        path=file_path,
-        filename=file_path.name,
-        media_type="application/octet-stream",
-    )
-
-
-@router.delete("/cleanup/{filename}")
-async def cleanup_translated_file(filename: str):
-    """Cleanup a translated file after download. Filename is sanitized to prevent path traversal."""
-    file_path, ok = _safe_output_path(filename)
-    if not ok or file_path is None:
-        raise HTTPException(status_code=400, detail="Invalid filename")
-    try:
-        if not file_path.exists():
-            raise HTTPException(status_code=404, detail="File not found")
-        file_handler.cleanup_file(file_path)
-        return {"message": f"File {file_path.name} deleted successfully"}
-    except HTTPException:
-        raise
-    except Exception as e:
-        logger.exception("Cleanup error")
-        raise HTTPException(
-            status_code=500, detail="Erreur lors de la suppression du fichier."
-        )
-
-
 @router.post("/extract-texts")
 async def extract_texts_from_document(
     file: UploadFile = File(..., description="Document file to extract texts from"),