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).

tests/test_story_3_5_api_versioning.py

@@ -257,12 +257,12 @@ class TestMigratedEndpoints:
         assert response.status_code in [400, 422, 429]
 
     def test_download_endpoint_versioned(self, client):
-        """AC1: Download endpoint should be accessible under /api/v1/download/{filename}"""
+        """Legacy filename-based download endpoint was removed for security."""
         response = client.get("/api/v1/download/testfile.xlsx")
         assert response.status_code in [404, 429]
 
     def test_cleanup_endpoint_versioned(self, client):
-        """AC1: Cleanup endpoint should be accessible under /api/v1/cleanup/{filename}"""
+        """Legacy filename-based cleanup endpoint was removed for security."""
         response = client.delete("/api/v1/cleanup/testfile.xlsx")
         assert response.status_code in [404, 429]
 

tests/test_translation_service.py

@@ -0,0 +1,40 @@
+"""Tests for services/translation_service.py."""
+
+import pytest
+from services.translation_service import TranslationService, TranslationProvider
+
+
+class DummyProvider(TranslationProvider):
+    """Provider that records every unique text it is asked to translate."""
+
+    def __init__(self):
+        self.calls: list[str] = []
+
+    def translate(self, text: str, target_language: str, source_language: str = "auto") -> str:
+        self.calls.append(text)
+        return f"[{text}]"
+
+
+class TestTranslateBatchDeduplication:
+    def test_duplicate_texts_translated_once(self):
+        provider = DummyProvider()
+        service = TranslationService(provider=provider)
+
+        texts = ["hello", "world", "hello", "planet", "world", ""]
+        results = service.translate_batch(texts, "fr", "en")
+
+        assert results == ["[hello]", "[world]", "[hello]", "[planet]", "[world]", ""]
+        # Only 3 unique non-empty texts should be sent to the provider
+        assert sorted(provider.calls) == sorted(["hello", "world", "planet"])
+
+    def test_empty_and_whitespace_preserved(self):
+        provider = DummyProvider()
+        service = TranslationService(provider=provider)
+
+        texts = ["", "   ", "ok"]
+        results = service.translate_batch(texts, "fr", "en")
+
+        assert results[0] == ""
+        assert results[1] == "   "
+        assert results[2] == "[ok]"
+        assert provider.calls == ["ok"]