feat: brain taxonomy — DB-backed folders/tags, sidebar, CRUD API

Backend:
- New Folder/Tag/ItemTag models with proper relational tables
- Taxonomy CRUD endpoints: list, create, rename, delete, merge tags
- Sidebar endpoint with folder/tag counts
- AI classification reads live folders/tags from DB, not hardcoded
- Default folders/tags seeded on first request per user
- folder_id FK on items for relational integrity

Frontend:
- Left sidebar with Folders/Tags tabs (like Karakeep)
- Click folder/tag to filter items
- "Manage" mode: add new folders/tags, delete existing
- Counts next to each folder/tag
- "All items" option to clear filter
- Replaces the old signal-strip cards

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

services/brain/app/main.py

@@ -6,6 +6,7 @@ from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
 from app.api.routes import router
+from app.api.taxonomy import router as taxonomy_router
 from app.config import DEBUG
 
 logging.basicConfig(
@@ -23,6 +24,7 @@ app = FastAPI(
 
 # No CORS — internal service only, accessed via gateway
 app.include_router(router)
+app.include_router(taxonomy_router)
 
 
 @app.on_event("startup")
@@ -30,6 +32,7 @@ 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
+    from app.models.taxonomy import Folder, Tag, ItemTag  # noqa: register taxonomy tables
 
     # Enable pgvector extension before creating tables
     async with engine.begin() as conn: