feat: brain service — self-contained second brain knowledge manager

Full backend service with:
- FastAPI REST API with CRUD, search, reprocess endpoints
- PostgreSQL + pgvector for items and semantic search
- Redis + RQ for background job processing
- Meilisearch for fast keyword/filter search
- Browserless/Chrome for JS rendering and screenshots
- OpenAI structured output for AI classification
- Local file storage with S3-ready abstraction
- Gateway auth via X-Gateway-User-Id header
- Own docker-compose stack (6 containers)

Classification: fixed folders (Home/Family/Work/Travel/Knowledge/Faith/Projects)
and fixed tags (28 predefined). AI assigns exactly 1 folder, 2-3 tags, title,
summary, and confidence score per item.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

services/brain/app/api/deps.py

@@ -0,0 +1,21 @@
+"""API dependencies — auth, database session."""
+
+from fastapi import Depends, Header, HTTPException
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.database import get_db
+
+
+async def get_user_id(
+    x_gateway_user_id: str = Header(None, alias="X-Gateway-User-Id"),
+) -> str:
+    """Extract authenticated user ID from gateway-injected header."""
+    if not x_gateway_user_id:
+        raise HTTPException(status_code=401, detail="Not authenticated")
+    return x_gateway_user_id
+
+
+async def get_db_session() -> AsyncSession:
+    """Provide an async database session."""
+    async for session in get_db():
+        yield session