fix: corriger 33 erreurs mypy (CI bloquant) + lien README (BUG-003, BUG-004)

BUG-003: annotations de types, gardes None sur get_user(), PdfReader: Any et import PROVIDERS manquant (bug latent main.py:4523). Etape mypy du CI rendue bloquante (etait advisory).

BUG-004: lien README.md -> docs/CONTRIBUTING.md corrige (+ DELIVERY_WORKFLOW.md), arbre projet mis a jour, parite README.fr.md.

Verifie: mypy 0 erreur, ruff OK, pytest 728 passed / 5 skipped, frontend OK, liens md OK.

backend/main.py

@@ -735,7 +735,7 @@ def _custom_openapi():
     return app.openapi_schema
 
 
-app.openapi = _custom_openapi
+app.openapi = _custom_openapi  # type: ignore[method-assign]
 
 # GZip compression — reduces bandwidth by ~70% for text responses
 # Custom wrapper: skip compression for SSE streams (/api/events)
@@ -2429,7 +2429,7 @@ def _list_backup_files(vault_name: str, relative_path: str) -> list[dict]:
 
     original_name = Path(relative_path).name
     prefix = original_name + "."
-    backups = []
+    backups: list[dict[str, Any]] = []
 
     try:
         dir_entries = list(backup_dir.iterdir())
@@ -3920,7 +3920,7 @@ async def api_vault_recent_files(
     if not dir_path.exists() or not dir_path.is_dir():
         raise HTTPException(status_code=404, detail=f"Directory not found: {dir}")
 
-    files = []
+    files: list[dict[str, Any]] = []
     dir_prefix = (dir or "").strip("/")
 
     try:
@@ -4022,7 +4022,7 @@ async def api_backups_list(
     current_user=Depends(require_auth),
 ):
     """List all backups across vaults, grouped by file."""
-    result = []
+    result: list[dict[str, Any]] = []
     try:
       for vault_name in index:
         if vault and vault_name != vault:
@@ -4357,7 +4357,7 @@ async def api_set_config(body: dict = Body(...), current_user=Depends(require_ad
 # AI API Keys — stored in data/api_keys.json, fallback to .env
 # ---------------------------------------------------------------------------
 
-from backend.ai import _read_ai_keys, get_ai_key
+from backend.ai import PROVIDERS, _read_ai_keys, get_ai_key
 
 AI_KEYS_FILE = Path("data/api_keys.json")