[codex] Remove persisted bounty summary column

[caydyan]

Summary

• Stop persisting one_sentence_summary on Bounty and NewBounty by marking the field as non-GORM state while keeping the JSON field available for existing clients.
• Drop the legacy one_sentence_summary column during database startup migrations when it still exists.
• Derive the legacy response value from title, matching the issue note that both values were duplicates.
• Remove migrated/seeded writes to the old column and add a schema test proving the summary field has no database column mapping.
• Refresh the generated database mock with GetBountyByUnlockCode so the current test packages compile.

Fixes #590

Validation

• go test ./db -run '^TestBountySummaryIsNotPersisted$' -count=1\n- go test ./db -run TestNonExistent -count=0\n- go test ./... -run TestNonExistent -count=0\n- git diff --check

[caydyan]

Current-head validation rerun on aab83a0:

• go test ./db -run TestBountySummaryIsNotPersisted -count=1 -v passed, including both Bounty and NewBounty schema subtests.
• go test ./... -run ^$ passed as a compile/no-test package sweep.
• git diff --check origin/master...HEAD passed.

This focused test does not require the local Postgres test database, so it ran cleanly in this environment.