feat: add Alembic for database migrations

Replace inline migration logic with proper Alembic migrations:
- 001: Initial schema (creates memories table with FTS)
- 002: Add multi-user and secrets columns (user_id, is_sensitive,
  vault_path, encrypted_content)

Migrations run automatically on app startup. Existing databases
are handled gracefully with IF NOT EXISTS / column existence checks.

docker/Dockerfile

@@ -2,8 +2,9 @@ FROM python:3.12-slim AS base
 
 WORKDIR /app
 
-COPY pyproject.toml README.md ./
+COPY pyproject.toml README.md alembic.ini ./
 COPY src/ src/
+COPY migrations/ migrations/
 
 RUN pip install --no-cache-dir ".[api]"