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/Dockerfile.api

@@ -0,0 +1,23 @@
+FROM python:3.12-slim
+
+WORKDIR /app
+
+RUN apt-get update && apt-get install -y --no-install-recommends libpq-dev && rm -rf /var/lib/apt/lists/*
+RUN pip install --no-cache-dir --upgrade pip
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
+
+RUN adduser --disabled-password --no-create-home appuser
+RUN mkdir -p /app/storage && chown -R appuser /app/storage
+
+COPY --chown=appuser app/ app/
+
+EXPOSE 8200
+ENV PYTHONUNBUFFERED=1
+
+HEALTHCHECK --interval=30s --timeout=5s --retries=3 \
+  CMD python3 -c "import urllib.request; urllib.request.urlopen('http://127.0.0.1:8200/api/health', timeout=3)" || exit 1
+
+USER appuser
+CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8200"]