fix(combos): purge combo-scoped proxy assignments when a combo is deleted (#14553)

[xiechimon]

What

Fixes #14553. Deleting a combo left orphaned proxy_assignments rows behind: the delete transaction in sqliteComboRepository.ts removed the combos row and the LKGP pins, but never touched the combo's proxy assignments or its proxy_scope_rotation cursor. The dashboard hides the dangling rows (the combo is gone from the list), yet GET /api/settings/proxies/assignments still reports an assignment for a scope_id that no longer exists, and the row counts toward active combo assignments.

How

• Inside the same delete transaction, drop every proxy_assignments row with scope = 'combo' for the deleted id and clear the combo's rotation cursor via clearRotationState (the existing shared helper from proxies/rotation). This mirrors the account-scope purge that providers/deletion.ts runs on connection delete (fix(backend): deleteProviderConnection leaves proxy_assignments orphan rows #9232).
• When rows were purged, bump the proxy registry generation so cached resolutions stop serving the deleted combo.
• The proxy itself stays registered: only the scope link is removed, so other combos/accounts still using that proxy are untouched. Deleting a combo without assignments is unchanged (no purge, no bump).

Verification

• New tests/unit/14553-combo-delete-purges-proxy-assignments.test.ts, red-green verified: all 4 cases fail on the base (assignment survives the delete) and pass with the fix. Cases: purge on delete, survivor combo + proxy row untouched, rotation cursor dropped for the doomed combo only, registry generation bumps.
• Adjacent regression: combo-delete LKGP (fix(backend): deleteCombo() leaves orphaned LKGP entries in key_value #12326), db-combos-crud, combo cache invalidation, db-proxies-crud/split, fix(backend): deleteProviderConnection leaves proxy_assignments orphan rows #9232 purge suite, pool rotation feat: native proxy-pool round-robin / egress IP rotation across multiple proxies #6365, combo-scope-proxy fix(resilience): combo routing ignores per-account proxy assignment #7149 suites, 87/87 green.
• prettier --check clean on both files, eslint clean, npm run typecheck:core exits 0.

No changelog fragment, following the precedent of #14300 and #14291.

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[diegosouzapw]

Clean fix, and mirroring the existing account-scope purge pattern from #9232 instead of
inventing something new was the right call. Ran the new test suite myself — all 4 cases pass,
including the "survivor combo is untouched" control. Merging as-is.