Update dependencies and GitHub Actions versions

[hardbyte]

Summary

Consolidates multiple Dependabot PRs into a single dependency update:

• GitHub Actions: actions/checkout v4→v6, actions/setup-python v5→v6, google-github-actions/auth v2→v3, google-github-actions/setup-gcloud v2→v3
• FastAPI 0.115.14→0.128.6 (widened constraint to >=0.115,<1.0)
• Ruff 0.4.10→0.15.0 (widened constraint to >=0.4.10,<1.0)
• setuptools 75.9.1→80.10.2 (widened constraint to >=75.6,<81)
• structlog 24.4.0→25.5.0 (widened constraint to >=24.1,<26)
• protobuf 6.33.4→6.33.5, locust 2.43.1→2.43.2 (transitive/dev)

Formatting changes from ruff 0.15 applied to 7 files (no logic changes).

Excluded: Python 3.14 Docker image (#570) — incompatible with python = ">=3.11,<3.14" constraint.

Supersedes

• Bump google-github-actions/setup-gcloud from 2 to 3 #569 (checkout v6)
• Bump ruff from 0.4.10 to 0.14.14 #571 (setup-python v6)
• Bump locust from 2.43.1 to 2.43.2 #572 (google-auth v3)
• Bump setuptools from 75.9.1 to 80.10.2 #573 (setuptools 80)
• Bump fastapi from 0.115.14 to 0.128.0 #574 (structlog 25)
• Bump structlog from 24.4.0 to 25.5.0 #575 (setup-gcloud v3)
• Bump protobuf from 6.33.4 to 6.33.5 #576 (ruff 0.15)
• Fix chat runtime option matching and add direct service calls #577 (fastapi 0.128)
• Bump setuptools from 75.9.1 to 78.1.1 #581 (setuptools 78 — superseded by Bump setuptools from 75.9.1 to 80.10.2 #573)

Test plan

• All 443 unit tests pass locally
• ruff check and ruff format --check pass
• CI integration tests pass
• Alembic migration tests pass