Phase 0 milestone 0.8: CI pipeline on Gitea Actions

Workflow moved to .gitea/workflows/ci.yml (Gitea is the CI host). Every
gate now passes locally, step-for-step; green on main is pending an
act_runner on gitea.doesworks.net (runs #1-#5 are queued "Waiting").

Gate fixes found by actually running them:
- pip-audit: `--disable-pip` without `-r` was a usage error, so the old
  step could never pass. Now `pip-audit --strict .` audits the runtime
  dependency set from pyproject.
- pip-audit then flagged ecdsa (PYSEC-2026-1325, no fix), a hard dep of
  python-jose. Swapped to PyJWT (HS256 only; 3-line change in
  auth/security.py). Flag to OWC.
- PyJWT warned the test JWT secret was 11 bytes. JWT_SECRET now requires
  >= 32 bytes (RFC 7518 s3.2) at settings load; dev/compose/test defaults
  updated. Test-only value carries an inline gitleaks:allow.
- bandit B101: assert in retention._count_past replaced with a raise.
- gitleaks: gitleaks-action needs a GitHub token/license; replaced with
  the pinned 8.30.1 binary, sha256-verified, scanning full history.

Added:
- SBOM: CycloneDX 1.6 from a clean runtime-only venv (39 components),
  uploaded as a CI artifact. YPMS-POAM-042.
- cyclonedx-bom in dev extras; CI outputs gitignored.

158 tests passing, 95.83% coverage; ruff, mypy, bandit, pip-audit,
gitleaks clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -60,3 +60,8 @@ logs/
 
 # Alembic scratch (real migrations are checked in under migrations/versions/)
 alembic/
+
+# CI outputs when run locally
+sbom.cdx.json
+gitleaks
+gitleaks.tgz