feat(08-05): decompose documents API monolith into documents/ package — CODE-02 CODE-08

- upload.py: request_upload_url, upload_document, confirm_upload (3 routes)
- crud.py: list_documents, get_document, patch_document, delete_document, classify_document (5 routes)
- content.py: stream_document_content + _parse_range (1 route)
- shared.py: _CLOUD_PROVIDERS, UploadUrlRequest, DocumentPatch (single definitions)
- __init__.py: router aggregator prefix=/api/documents, 9 routes total
- documents.py monolith deleted; all URL paths unchanged
- list_documents registered on parent router to avoid FastAPI empty-path/prefix check
- get_storage_backend_for_document re-exported for test monkeypatching compatibility

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/api/documents/__init__.py

@@ -0,0 +1,28 @@
+"""Document API package — router aggregator.
+
+Aggregates upload_router, crud_router, and content_router under a single
+APIRouter with prefix="/api/documents". This is the ONLY file in the package
+that sets the prefix — sub-routers declare no prefix (D-04).
+
+main.py import unchanged after decomposition:
+  from api.documents import router as documents_router
+  app.include_router(documents_router)
+"""
+from fastapi import APIRouter
+
+from api.documents.upload import router as upload_router
+from api.documents.crud import router as crud_router, list_documents
+from api.documents.content import router as content_router
+# Re-export for test monkeypatching: tests patch api.documents.extract_and_classify.delay
+# (same Celery task object as upload.py uses — patching .delay affects all callers)
+from tasks.document_tasks import extract_and_classify  # noqa: F401
+from storage import get_storage_backend_for_document  # noqa: F401
+
+router = APIRouter(prefix="/api/documents", tags=["documents"])
+router.include_router(upload_router)
+# list_documents registered directly on parent: FastAPI 0.100+ disallows include_router
+# when the include prefix AND route path are both empty strings (our sub-router pattern
+# uses no prefix per D-04, and the list endpoint has path="").
+router.add_api_route("", list_documents, methods=["GET"])
+router.include_router(crud_router)
+router.include_router(content_router)