feat: add PostgreSQL initialization script for AI stack

Created database initialization script following the core stack pattern.
The script automatically creates required databases on first initialization:
- openwebui: Open WebUI application database
- litellm: LiteLLM proxy database for API key management and tracking

Changes:
- Created ai/postgres/init/01-init-databases.sh
- Mounted init directory in ai_postgres service
- Added automatic privilege grants to AI_DB_USER

Note: Init script only runs on first database creation when volume is empty.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

ai/compose.yaml

@@ -13,6 +13,7 @@ services:
       POSTGRES_INITDB_ARGS: --auth-host=scram-sha-256
     volumes:
       - ai_postgres_data:/var/lib/postgresql/data
+      - ./postgres/init:/docker-entrypoint-initdb.d
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U ${AI_DB_USER}"]
       interval: 30s