fix: brain service — pgvector extension init, meili key length

Brain service fully operational:
- All 6 containers running
- API responds on /api/health
- Worker processes items (fetch, screenshot, classify, index)
- Tested: link creation, background processing, asset archival

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

services/brain/app/main.py

@@ -27,11 +27,13 @@ app.include_router(router)
 
 @app.on_event("startup")
 async def startup():
+    from sqlalchemy import text as sa_text
     from app.database import engine, Base
     from app.models.item import Item, ItemAsset, AppLink  # noqa: import to register models
 
-    # Create tables if they don't exist
+    # Enable pgvector extension before creating tables
     async with engine.begin() as conn:
+        await conn.execute(sa_text("CREATE EXTENSION IF NOT EXISTS vector"))
         await conn.run_sync(Base.metadata.create_all)
 
     # Ensure Meilisearch index exists