feat(notifications): per-recipient mute + List-Unsubscribe (closes #297)

[cristim]

Summary

• Adds migration 000061 (muted_recipients table keyed on (recipient_email, scope)) for per-recipient, per-scope opt-out persistence.
• Adds GET /api/notifications/unsubscribe?token=...&email=...&scope=... (AuthPublic) that verifies an HMAC-SHA256 signed token, upserts the mute row, and returns a minimal HTML confirmation page (RFC 8058 one-click flow).
• Wires mute checking and List-Unsubscribe / List-Unsubscribe-Post headers into SendPurchaseApprovalRequest; the SES send is skipped silently when the recipient is muted, and the headers are attached when unsubscribeBaseURL is configured.

Design notes

• Token mechanism: HMAC-SHA256(NOTIFICATION_MUTE_SECRET, lower(email)+"|"+scope) in pkg/common.DeriveMuteToken; compared with hmac.Equal (constant time). Dev/test fallback key used when env var is unset.
• Mute is fail-open: a DB error during the mute check is logged and the email is sent, so a transient outage never blocks approval notifications.
• Scope whitelist enforced at the handler boundary (purchase_approvals / ri_exchange_approvals).
• PII: emails are logged only as us***@example.com (redacted).

Test plan

• go test github.com/LeanerCloud/CUDly/internal/email/... github.com/LeanerCloud/CUDly/internal/api/... github.com/LeanerCloud/CUDly/pkg/common/... passes (1867 tests)
• TestUnsubscribeHandler_Success - valid signed token stores mute row, returns HTML page
• TestUnsubscribeHandler_ForgedToken_Returns401 - bad token rejected
• TestSendPurchaseApprovalRequest_MutedRecipient_NoSESCall - muted address skips SES call
• TestSendPurchaseApprovalRequest_NotMuted_SendsEmail - normal path unchanged
• TestSendPurchaseApprovalRequest_MuteCheckError_FailOpen - DB error doesn't block send
• RFC 8058 compliance: List-Unsubscribe carries angle-bracketed HTTPS URL; List-Unsubscribe-Post: List-Unsubscribe=One-Click present when base URL configured

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: a9063678-47b8-4b27-b6d2-0e3b281e0f45

📥 Commits

Reviewing files that changed from the base of the PR and between 4956d66 and 958203b.

📒 Files selected for processing (20)

• internal/analytics/collector_test.go
• internal/api/handler_notifications.go
• internal/api/handler_notifications_test.go
• internal/api/middleware.go
• internal/api/mocks_test.go
• internal/api/router.go
• internal/config/interfaces.go
• internal/config/store_postgres.go
• internal/database/postgres/migrations/000061_muted_recipients.down.sql
• internal/database/postgres/migrations/000061_muted_recipients.up.sql
• internal/email/interfaces.go
• internal/email/mute_test.go
• internal/email/sender.go
• internal/email/templates.go
• internal/purchase/mocks_test.go
• internal/scheduler/scheduler_overrides_test.go
• internal/scheduler/scheduler_suppressions_test.go
• internal/scheduler/scheduler_test.go
• internal/server/test_helpers_test.go
• pkg/common/tokens.go

✨ Finishing Touches

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• Create PR with unit tests
• Commit unit tests in branch fix/297-wave9

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