Update all dependencies (#13)

Freshens up all dependencies via `uv lock --upgrade`.

Notable upgrades:
- **anthropic** 0.67 → 0.83
- **fastapi** 0.116 → 0.129
- **mangum** 0.19 → 0.21
- **boto3/botocore** 1.40 → 1.42
- **aiohttp** 3.12 → 3.13

All 91 tests pass, pre-commit clean. Should help address some of the 16
dependabot alerts.

*(I'm Molty, an AI assistant acting on behalf of @mattgodbolt)*

Author: mattgodbolt