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/pdf_reader.py

@@ -6,6 +6,7 @@ import os
 from concurrent.futures import ThreadPoolExecutor
 from concurrent.futures import TimeoutError as FuturesTimeout
 from pathlib import Path
+from typing import Any
 
 logger = logging.getLogger(__name__)
 
@@ -16,7 +17,7 @@ PDF_MAX_SIZE_MB: int = int(os.environ.get("OBSIGATE_PDF_MAX_SIZE_MB", "50"))
 PDF_EXTRACT_TIMEOUT: float = float(os.environ.get("OBSIGATE_PDF_EXTRACT_TIMEOUT", "30"))
 
 PDF_READER: str = "pypdf"
-PdfReader = None  # type: ignore
+PdfReader: Any = None
 try:
     import fitz  # pymupdf
     PDF_READER = "pymupdf"
@@ -90,7 +91,7 @@ def extract_pdf_metadata(file_path: Path) -> dict:
             info["title"] = meta.get("title", "")
             info["author"] = meta.get("author", "")
             doc.close()
-        else:
+        elif PdfReader is not None:
             reader = PdfReader(str(file_path))
             info["pages"] = len(reader.pages)
             meta = reader.metadata or {}
@@ -159,6 +160,8 @@ def _extract_pymupdf(file_path: Path, max_chars: int) -> str:
 
 
 def _extract_pypdf(file_path: Path, max_chars: int) -> str:
+    if PdfReader is None:
+        return ""
     reader = PdfReader(str(file_path))
     parts = []
     total = 0