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/migrations/001_init.sql

@@ -0,0 +1,56 @@
+-- Brain service schema — PostgreSQL + pgvector
+-- This is a reference migration. Tables are auto-created by SQLAlchemy on startup.
+
+CREATE EXTENSION IF NOT EXISTS vector;
+
+CREATE TABLE IF NOT EXISTS items (
+    id UUID PRIMARY KEY,
+    user_id VARCHAR(64) NOT NULL,
+    type VARCHAR(32) NOT NULL DEFAULT 'link',
+    title TEXT,
+    url TEXT,
+    raw_content TEXT,
+    extracted_text TEXT,
+    folder VARCHAR(64),
+    tags TEXT[],
+    summary TEXT,
+    confidence FLOAT,
+    metadata_json JSONB DEFAULT '{}',
+    processing_status VARCHAR(32) NOT NULL DEFAULT 'pending',
+    processing_error TEXT,
+    embedding vector(1536),
+    created_at TIMESTAMP NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMP NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS ix_items_user_status ON items(user_id, processing_status);
+CREATE INDEX IF NOT EXISTS ix_items_user_folder ON items(user_id, folder);
+CREATE INDEX IF NOT EXISTS ix_items_created ON items(created_at);
+
+-- HNSW index for fast approximate nearest neighbor search
+CREATE INDEX IF NOT EXISTS ix_items_embedding ON items
+    USING hnsw (embedding vector_cosine_ops)
+    WITH (m = 16, ef_construction = 64);
+
+CREATE TABLE IF NOT EXISTS item_assets (
+    id UUID PRIMARY KEY,
+    item_id UUID NOT NULL REFERENCES items(id) ON DELETE CASCADE,
+    asset_type VARCHAR(32) NOT NULL,
+    filename VARCHAR(512) NOT NULL,
+    content_type VARCHAR(128),
+    size_bytes INTEGER,
+    storage_path VARCHAR(1024) NOT NULL,
+    created_at TIMESTAMP NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS ix_item_assets_item ON item_assets(item_id);
+
+CREATE TABLE IF NOT EXISTS app_links (
+    id UUID PRIMARY KEY,
+    item_id UUID NOT NULL REFERENCES items(id) ON DELETE CASCADE,
+    app VARCHAR(64) NOT NULL,
+    app_entity_id VARCHAR(128) NOT NULL,
+    created_at TIMESTAMP NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS ix_app_links_item ON app_links(item_id);