Add feature phase tickets route

[cybercraftsolutionsllc]

Summary

• add GET /features/{feature_uuid}/phase/{phase_uuid}/tickets
• validate auth, feature existence, and phase existence before returning phase tickets
• update route coverage and generated Swagger docs for the new endpoint

Fixes #1982
Addresses #1983

Validation

• go test ./routes -run '^TestFeatureRoutes$' -count=1
• go test ./routes -run '^$' -count=1
• git diff --check
• Get-Content -Raw docs\swagger.json | ConvertFrom-Json | Out-Null

Known existing blocker

• go test ./handlers -run '^$' -count=1 is still blocked by the existing stale mock issue: mocks.Database does not implement db.Database because it is missing GetBountyByUnlockCode.