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/indexer.py

@@ -303,27 +303,27 @@ def _decompress_excalidraw(compressed: str) -> dict[str, Any] | None:
         if index > length:
             return None
 
-        c = _read_bits(num_bits)
-        if c == 0:
+        code = _read_bits(num_bits)
+        if code == 0:
             dictionary.append(chr(_read_bits(8)))
             dict_size += 1
-            c = dict_size - 1
+            code = dict_size - 1
             enlarge_in -= 1
-        elif c == 1:
+        elif code == 1:
             dictionary.append(chr(_read_bits(16)))
             dict_size += 1
-            c = dict_size - 1
+            code = dict_size - 1
             enlarge_in -= 1
-        elif c == 2:
+        elif code == 2:
             break  # end of stream
 
         if enlarge_in == 0:
             enlarge_in = 1 << num_bits
             num_bits += 1
 
-        if c < len(dictionary) and dictionary[c]:
-            entry = dictionary[c]
-        elif c == dict_size:
+        if code < len(dictionary) and dictionary[code]:
+            entry = dictionary[code]
+        elif code == dict_size:
             entry = w + w[0]
         else:
             return None
@@ -461,22 +461,20 @@ def _scan_vault(vault_name: str, vault_path: str, vault_cfg: dict[str, Any] | No
             modified = datetime.fromtimestamp(stat.st_mtime, tz=timezone.utc).isoformat()
 
             # PDF handling — special path (binary, uses pdf_reader)
+            tags: list[str] = []
             if ext == ".pdf":
                 from backend.pdf_reader import extract_pdf_metadata, extract_pdf_text
                 raw = extract_pdf_text(fpath, max_chars=100000)
                 pdf_meta = extract_pdf_metadata(fpath)
                 title = pdf_meta.get("title") or fpath.stem.replace("-", " ").replace("_", " ")
                 content_preview = raw[:200].strip()
-                tags: list[str] = []
             elif ext == ".excalidraw" or fpath.name.lower().endswith(".excalidraw.md"):
                 raw = fpath.read_text(encoding="utf-8", errors="replace")
                 raw = extract_excalidraw_indexable(raw)
-                tags: list[str] = []
                 title = fpath.stem.replace(".excalidraw", "").replace("-", " ").replace("_", " ")
                 content_preview = raw[:200].strip()
             else:
                 raw = fpath.read_text(encoding="utf-8", errors="replace")
-                tags: list[str] = []
                 title = fpath.stem.replace("-", " ").replace("_", " ")
                 content_preview = raw[:200].strip()