fix(db): write canonical 'canceled' on all cancel paths + cleanup filter (follow-up to #1277)

[cristim]

Summary

Follow-up to #1277 (expand-contract rename 'cancelled'->'canceled'). Three categories of defects verified on current main and fixed.

Defect 1 (medium, latent deploy trap): Every cancel write path still emitted status='cancelled' and wrote cancelled_by instead of the canonical status='canceled' / canceled_by. If the contract migration #1278 narrows the CHECK to drop 'cancelled' before these writers switch, every session/RI-exchange cancel path fails with check_violation (SQLSTATE 23514).

Paths fixed:

• store_postgres.go: CancelExecutionAtomic, CancelScheduledExecutionAtomic - now write status=StatusCanceled via $3 param and canceled_by column
• store_postgres.go: CancelAllPendingExchanges, CancelPendingExchangesByOrigin (both branches) - now write status=StatusCanceled via $1 param
• handler_ri_exchange.go rejectRIExchange (~line 1372) - uses config.StatusCanceled for TransitionRIExchangeStatus call and response body

Defect 2 (low, silent cleanup miss): CleanupOldExecutions filtered status IN ('completed', 'cancelled') - omitting 'canceled', so rows written by new code after #1277 would never be terminal-cleaned.

Fixed: added 'canceled' to the IN clause; updated godoc comment.

Regression tests added

8 new pgxmock tests in store_postgres_pgxmock_test.go:

• TestPGXMock_CancelExecutionAtomic_WritesCanonicalStatus - asserts $3=StatusCanceled and canceled_by column
• TestPGXMock_CancelScheduledExecutionAtomic_WritesCanonicalStatus - same for scheduled path
• TestPGXMock_CancelAllPendingExchanges_WritesCanonicalStatus - asserts $1=StatusCanceled
• TestPGXMock_CancelPendingExchangesByOrigin_Standalone_WritesCanonicalStatus
• TestPGXMock_CancelPendingExchangesByOrigin_Ladder_WritesCanonicalStatus
• TestPGXMock_CleanupOldExecutions_IncludesCanonicalStatus - regex pins both 'cancelled' and 'canceled' in SQL

4 existing tests updated to reflect the canonical spelling (were asserting "cancelled" as the transition target).

Test plan

• go build ./... clean
• go vet ./internal/config/... ./internal/api/... clean
• go test ./internal/config/... - 650 passed
• go test ./internal/api/... - 1767 passed
• go test ./internal/... - 5020 passed in 24 packages
• CI green
• @coderabbitai review

@coderabbitai review

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 1bc6fd18-e05f-4344-ad67-9a75653a3ff0

📥 Commits

Reviewing files that changed from the base of the PR and between 1d9f1b8 and fa6ae3f.

📒 Files selected for processing (6)

• internal/api/coverage_extras_test.go
• internal/api/handler_ri_exchange.go
• internal/api/handler_ri_exchange_test.go
• internal/api/router_handlers_test.go
• internal/config/store_postgres.go
• internal/config/store_postgres_pgxmock_test.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/1277-followup-canonical-cancel-writes

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