feat: migrate doc-service to use storage-service for file I/O (Phase 2)

- storage.py: replace aiofiles filesystem ops with httpx calls to
  storage-service PUT/GET/DELETE /objects/documents/{key}
- Document model: rename file_path → storage_key (plain object key, no path prefix)
- Migration 0008: ALTER COLUMN + data migration strips /data/documents/ prefix
- documents.py: update upload, delete, download endpoints; _extract_pdf_text
  now takes bytes (pdfplumber.open(BytesIO)) instead of a filesystem path
- file_watcher.py: store storage_key instead of file_path on ingestion
- doc-service config: add STORAGE_SERVICE_URL env var

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

features/doc-service/app/models/document.py

@@ -13,7 +13,7 @@ class Document(Base):
     id: Mapped[str] = mapped_column(String, primary_key=True, default=lambda: str(uuid.uuid4()))
     user_id: Mapped[str] = mapped_column(String, nullable=False, index=True)
     filename: Mapped[str] = mapped_column(String, nullable=False)
-    file_path: Mapped[str] = mapped_column(String, nullable=False)
+    storage_key: Mapped[str] = mapped_column(String, nullable=False)
     file_size: Mapped[int] = mapped_column(Integer, nullable=False)
     status: Mapped[str] = mapped_column(String, nullable=False, default="pending")
     title: Mapped[str | None] = mapped_column(String(500), nullable=True)