feat(documents): add text pdf ingestion with heading reconstruction

Why:
- Support Text PDFs in the same DocumentTree/markdown contract as DOCX.

Changes:
- PyMuPDF parser, text-layer gate, shared heading heuristics, upload dispatch for .pdf.

Impact:
- Scanned/image PDFs are rejected at upload; requires pymupdf installed.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/core/config.py

@@ -35,5 +35,9 @@ class Settings(BaseSettings):
     # Database
     database_url: str = "sqlite:///./data/chunking_benchmark.db"
 
+    # Text PDF gate (reject Scanned PDFs with near-empty text layer)
+    pdf_min_total_chars: int = 100
+    pdf_min_median_chars_per_page: int = 40
+
 
 settings = Settings()