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/analyze_i18n.py

@@ -0,0 +1,20 @@
+import re
+from pathlib import Path
+from collections import Counter
+
+content = Path("frontend/src/lib/i18n.tsx").read_text(encoding="utf-8")
+
+# Extract the en block (from "en: {" up to "\n  },\n  fr:")
+m = re.search(r'en:\s*\{(.*?)\n\s*\},\s*\n\s*fr:', content, re.DOTALL)
+if not m:
+    print("Could not find en block")
+    raise SystemExit(1)
+
+en_block = m.group(1)
+# Find all top-level keys in the en dictionary
+keys = re.findall(r'"([a-zA-Z0-9_\-\.]+)":', en_block)
+namespaces = Counter(k.split('.')[0] for k in keys)
+
+for ns, count in namespaces.most_common():
+    print(f"{ns}: {count}")
+print(f"Total keys: {len(keys)}")

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")

scripts/split_i18n.py

@@ -0,0 +1,107 @@
+"""
+Split frontend/src/lib/i18n.tsx into per-locale, per-namespace JSON files.
+
+Generates:
+  frontend/src/lib/i18n/messages/<locale>/<namespace>.json
+  frontend/src/lib/i18n/messages/index.json (manifest)
+"""
+
+import json
+import re
+from pathlib import Path
+from collections import defaultdict
+
+ROOT = Path(__file__).parent.parent
+SOURCE = ROOT / "frontend" / "src" / "lib" / "i18n.tsx"
+OUT_DIR = ROOT / "frontend" / "src" / "lib" / "i18n" / "messages"
+
+content = SOURCE.read_text(encoding="utf-8")
+
+def find_locale_blocks(text: str) -> list[tuple[str, str]]:
+    """Find each locale block using brace matching."""
+    blocks = []
+    pattern = re.compile(r'\n\s*([a-z]{2}):\s*\{')
+    for match in pattern.finditer(text):
+        locale = match.group(1)
+        start = match.end() - 1  # position of the opening '{'
+        brace_count = 0
+        in_string = False
+        escape = False
+        i = start
+        while i < len(text):
+            ch = text[i]
+            if escape:
+                escape = False
+            elif ch == "\\":
+                escape = True
+            elif ch == '"':
+                in_string = not in_string
+            elif not in_string:
+                if ch == "{":
+                    brace_count += 1
+                elif ch == "}":
+                    brace_count -= 1
+                    if brace_count == 0:
+                        blocks.append((locale, text[start + 1 : i]))
+                        break
+            i += 1
+    return blocks
+
+def parse_block(block: str) -> dict[str, str]:
+    """Parse key: value pairs from a locale block. Values may be concatenated strings."""
+    messages = {}
+    # Match "key": value,  where value is a string literal possibly followed by + "..."
+    entry_pattern = re.compile(
+        r'"([a-zA-Z0-9_\-\.]+)":\s*((?:"(?:[^"\\]|\\.)*"\s*(?:\+\s*)?)+)',
+        re.DOTALL,
+    )
+    for match in entry_pattern.finditer(block):
+        key = match.group(1)
+        raw = match.group(2)
+        parts = re.findall(r'"((?:[^"\\]|\\.)*)"', raw)
+        value = "".join(parts)
+        messages[key] = value
+    return messages
+
+OUT_DIR.mkdir(parents=True, exist_ok=True)
+
+manifest: dict[str, list[str]] = defaultdict(list)
+all_namespaces: set[str] = set()
+
+for locale, block in find_locale_blocks(content):
+    messages = parse_block(block)
+    by_namespace: dict[str, dict[str, str]] = defaultdict(dict)
+    for key, value in messages.items():
+        namespace = key.split(".")[0]
+        by_namespace[namespace][key] = value
+        all_namespaces.add(namespace)
+
+    locale_dir = OUT_DIR / locale
+    locale_dir.mkdir(parents=True, exist_ok=True)
+    for namespace, msgs in by_namespace.items():
+        (locale_dir / f"{namespace}.json").write_text(
+            json.dumps(msgs, indent=2, ensure_ascii=False) + "\n",
+            encoding="utf-8",
+        )
+        if namespace not in manifest[locale]:
+            manifest[locale].append(namespace)
+
+# Write manifest
+manifest = {loc: sorted(manifest[loc]) for loc in sorted(manifest)}
+(OUT_DIR / "index.json").write_text(
+    json.dumps(
+        {
+            "locales": list(manifest.keys()),
+            "namespaces": sorted(all_namespaces),
+            "manifest": manifest,
+        },
+        indent=2,
+        ensure_ascii=False,
+    )
+    + "\n",
+    encoding="utf-8",
+)
+
+print(f"Wrote namespace files for {len(manifest)} locales")
+print(f"Namespaces: {len(all_namespaces)}")
+print(f"Total keys (en): {sum(len(manifest[loc]) for loc in manifest)}")