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/services/file_watcher.py

@@ -3,7 +3,7 @@ File-system watcher for the watch directory.
 
 Uses the watchdog library to monitor a configured directory for new PDF files.
 When a PDF is detected, it is automatically ingested into the document service
-(copied to /data/documents, a DB record is created, and the AI pipeline runs).
+(uploaded to storage-service, a DB record is created, and the AI pipeline runs).
 
 Key design decisions:
 - No-remove policy: on_deleted and on_moved events are intentionally ignored.
@@ -82,13 +82,13 @@ async def ingest_file(path_str: str, watch_root: Path, config: dict) -> None:
             logger.warning("[watcher] Cannot read %s: %s", path_str, exc)
             return
 
-        # Save a copy to /data/documents/watch/{doc_id}.pdf
+        # Upload to storage-service under documents/watch/{doc_id}.pdf
         doc_id = existing.id if existing is not None else str(uuid.uuid4())
-        dest = await save_upload(file_data, WATCH_USER_ID, doc_id)
+        storage_key = await save_upload(file_data, WATCH_USER_ID, doc_id)
 
         if existing is not None:
             # Re-ingest a previously failed document
-            existing.file_path = str(dest)
+            existing.storage_key = storage_key
             existing.file_size = len(file_data)
             existing.status = "pending"
             existing.error_message = None
@@ -100,7 +100,7 @@ async def ingest_file(path_str: str, watch_root: Path, config: dict) -> None:
                 source="watch",
                 watch_path=path_str,
                 filename=path.name,
-                file_path=str(dest),
+                storage_key=storage_key,
                 file_size=len(file_data),
                 status="pending",
             )