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/core/config.py

@@ -7,6 +7,7 @@ class Settings(BaseSettings):
     DATA_DIR: str = "/data/documents"
     CONFIG_PATH: str = "/config/doc_service_config.json"
     AI_SERVICE_URL: str = "http://ai-service:8010"
+    STORAGE_SERVICE_URL: str = "http://storage-service:8020"
 
     class Config:
         env_file = ".env"