fix(db): drop the 2 orphaned scoring-intelligence tables from #4619

[JSONbored]

What

Completes #4619's deferred scope. PR #4666 already removed the other 12 confirmed-dead exports; the
issue_quality_reports/registry_drift_events DB-table drop was deliberately held back pending
confirmation there's no production data, since that can't be verified from the codebase alone.

Confirmed directly against the live production database: both tables have zero rows. Safe to drop.

Both were scaffolded in migration 0004_scoring_intelligence.sql alongside 6 sibling tables
(scoring_model_snapshots, score_previews, contributor_evidence, contributor_scoring_profiles,
burden_forecasts, bounty_lifecycle_events) that all got wired with real read+write paths — these two
alone never did. The "issue quality report" concept now lives entirely in the generic signal_snapshots
cache via src/services/issue-quality.ts instead.

Changes

• migrations/0141_drop_orphaned_scoring_intelligence_tables.sql — DROP TABLE IF EXISTS for both.
• src/db/schema.ts — removed the two Drizzle table definitions.
• src/db/repositories.ts — removed upsertIssueQualityReport/persistRegistryDriftEvents (zero callers
  anywhere) and their now-unused imports.
• src/types.ts — removed IssueQualityReportRecord/RegistryDriftEventRecord (only ever referenced by
  the two removed functions).
• test/unit/db-persistence.test.ts — removed the two functions' round-trip coverage from a shared test
  that also exercises other, still-live persistence helpers; kept everything else in that test intact.

Verified zero remaining references to any of these identifiers anywhere in src/, test/, or packages/.

Testing

npm run db:migrations:check and npm run db:schema-drift:check both pass clean (144 contiguous
migrations, schema.ts matches migrations/). test/unit/db-persistence.test.ts (24 tests) passes.
Typecheck clean.

Fixes #4619

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.14%. Comparing base (347ada8) to head (e99ad44).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4676   +/-   ##
=======================================
  Coverage   94.14%   94.14%           
=======================================
  Files         435      435           
  Lines       38576    38566   -10     
  Branches    14060    14060           
=======================================
- Hits        36316    36308    -8     
+ Misses       1601     1599    -2     
  Partials      659      659           

Files with missing lines	Coverage Δ
src/db/repositories.ts	96.72% <ø> (-0.01%)	⬇️
src/db/schema.ts	73.07% <ø> (+0.34%)	⬆️
src/types.ts	100.00% <ø> (ø)

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