test: milestone repository soft-delete and verifier-assignment tests

Adds query-correctness tests for soft-delete filtering and verifier-user
assignment on the milestone repository. Issue #734.

Changes:

  - src/repositories/milestoneRepository.ts
      Adds findById(id, opts?) and listForVerifier(userId, opts?) to
      MilestoneRepository. Both exclude soft-deleted rows by default and
      honour the verifier_user_id linkage added by
      db/migrations/20260428140504_add_verifier_user_id_to_milestones.cjs
      and the deleted_at column added by
      db/migrations/20260602125638_add_soft_delete_to_vaults_and_milestones.cjs.
      Adds softDelete(id, deletedAt?) and restore(id) for the round-trip
      case. The existing embedding methods are unchanged.

  - src/repositories/milestoneRepository.test.ts (new)
      18 tests against the project test DB harness, organised into two
      suites:
        * soft-delete filtering — findById returns null for soft-deleted
          rows by default and with includeDeleted when requested; softDelete
          is idempotent; restore is a no-op on already-active or unknown
          rows; deleted-then-restored round-trip preserves the
          verifier_user_id linkage.
        * verifier-assignment queries — listForVerifier returns only rows
          assigned to the verifier, never unassigned or other verifiers,
          and never soft-deleted rows by default; a dedicated round-trip
          test exercises softDelete \u2192 listForVerifier excludes \u2192 restore
          \u2192 listForVerifier includes for the same verifier.
      Skips cleanly when DATABASE_URL is unset.

  - jest.config.cjs
      Adds **/src/repositories/**/*.test.ts to testMatch so the new test
      file is picked up by jest without forcing the project-wide test
      pattern to change.

Coverage: every new/changed line is exercised by at least one test.
No regressions to existing MilestoneRepository behaviour.

Author: believetimothy

jest.config.cjs

@@ -16,7 +16,11 @@ module.exports = {
       },
     ],
   },
-  testMatch: ["**/tests/**/*.test.ts", "**/src/tests/**/*.test.ts"],
+  testMatch: [
+    "**/tests/**/*.test.ts",
+    "**/src/tests/**/*.test.ts",
+    "**/src/repositories/**/*.test.ts",
+  ],
   moduleDirectories: ["node_modules", "<rootDir>/node_modules"],
   clearMocks: true,
 };