fix(api): preserve worker claims when pausing purchases

[cristim]

Pausing an execution already claimed by a worker changed it from running to paused. Resuming made it pending again, allowing a second worker to enter the purchase while the first was still active. Restrict Pause to pending executions so the atomic status transition preserves the worker's claim. Pending pause/resume remains supported; ineligible states return 409, now documented on the endpoint.

Closes #223

Independent gpt-6-astra review approved exact commit 1c5a14964444ed1015dee47346e875809e616ac8 under the authorized local review alternative, with no actionable findings in both implementation rounds or final committed-source review.

Fresh independent PostgreSQL 16 race-enabled tests exercised the production handler, store and Manager.ProcessMessage with synthetic provider, credential, auth-session and email boundaries. They assert both claim/pause orderings, unchanged claim attribution, one provider purchase and one persisted history row, queued redelivery safety, and legitimate pending pause/resume. The committed regression against the parent handler failed on two provider entries and two history rows; removing pending from the allowed set failed the valid pause control. Four connected scenarios also passed three independent race-enabled repetitions before commit.

The full untagged race suite, backend build and normal installed hooks passed. No live-cloud purchase was performed. CI is a separate publication gate.

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📥 Commits

Reviewing files that changed from the base of the PR and between 6d9a70f and 1c5a149.

📒 Files selected for processing (6)

• internal/api/handler_purchases.go
• internal/api/handler_purchases_pause_claim_integration_test.go
• internal/api/handler_purchases_test.go
• internal/api/handler_test.go
• internal/api/openapi.yaml
• internal/api/router_660_permission_flips_test.go

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