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/documents/routes.py

@@ -1,8 +1,8 @@
 """Document and strategy API routes.
 
 Endpoints:
-  POST   /documents              Upload a .docx file
-  POST   /documents/{id}/process  Run chunking strategies (stub until Phase 2)
+  POST   /documents              Upload a .docx / .doc / .pdf file
+  POST   /documents/{id}/process  Run chunking strategies
   DELETE /documents/{id}          Remove document + vectors
   GET    /strategies              List available chunking strategies
 """
@@ -10,7 +10,6 @@ Endpoints:
 from fastapi import APIRouter, File, UploadFile
 
 from src.core.exceptions import DocumentProcessingError
-from src.core.models import PaginatedResponse, StrategyName
 from src.documents.models import (
     DeleteResponse,
     DocumentResponse,
@@ -20,6 +19,7 @@ from src.documents.models import (
     StrategiesResponse,
     StrategyInfo,
 )
+from src.documents.parser import SUPPORTED_SUFFIXES
 from src.documents import service
 
 router = APIRouter()
@@ -39,11 +39,15 @@ async def list_documents(offset: int = 0, limit: int = 50):
 
 @router.post("/documents", response_model=DocumentResponse, status_code=201)
 async def upload_document(file: UploadFile = File(...)):
-    """Upload a .docx file. Parses it, stores the document tree in SQLite."""
+    """Upload a document. Parses it, stores the document tree in SQLite."""
     if not file.filename:
         raise DocumentProcessingError("No filename provided")
-    if not file.filename.lower().endswith((".docx", ".doc")):
-        raise DocumentProcessingError("Only .docx and .doc files are supported")
+
+    lower = file.filename.lower()
+    if not any(lower.endswith(s) for s in SUPPORTED_SUFFIXES):
+        raise DocumentProcessingError(
+            f"Only {', '.join(SUPPORTED_SUFFIXES)} files are supported"
+        )
 
     content = await file.read()
     if not content:
@@ -61,10 +65,7 @@ async def upload_document(file: UploadFile = File(...)):
 
 @router.post("/documents/{doc_id}/process", response_model=ProcessResponse)
 async def process_document(doc_id: str, request: ProcessRequest):
-    """Run selected chunking strategies on an uploaded document.
-
-    Currently returns stub results until Phase 2 implements the strategies.
-    """
+    """Run selected chunking strategies on an uploaded document."""
     return service.process_document(doc_id, request)