refactor: unify webhook delivery pipeline and migrate to Redis

[ericfitz]

Summary

Complete the webhook delivery unification started in #194. This issue covers Phases 2 and 3 of the design.

Design spec: docs/superpowers/specs/2026-03-29-webhook-delivery-unification-design.md

Background

Issue #194 (Phase 1) unified the addon invocation webhook payload and headers to match resource-change webhooks, and renamed invocation_id to delivery_id. This issue completes the convergence by building a unified Redis-backed delivery pipeline.

Design Decisions

• All delivery state moves to Redis (replacing both Postgres webhook_deliveries and Redis AddonInvocation stores)
• Any webhook delivery can use X-TMI-Callback: async for bidirectional status callbacks (not just addon invocations)
• Addon invocations emit events to Redis Streams and flow through the same delivery worker as resource-change events
• New GET /webhook-deliveries/{id} endpoint supports JWT or HMAC authentication
• New POST /webhook-deliveries/{id}/status endpoint replaces POST /invocations/{id}/status

Redis TTLs

• pending / in_progress: 4 hours
• failed / completed / delivered: 7 days

Phase 2: Unified Redis Delivery Model + Migrate Addon Invocations

• Create WebhookDeliveryRedisStore with unified delivery record
• Add POST /webhook-deliveries/{id}/status endpoint (replaces POST /invocations/{id}/status)
• Add GET /webhook-deliveries/{id} endpoint with JWT+HMAC dual auth (replaces GET /invocations/{id})
• Remove GET /invocations list endpoint
• Modify POST /addons/{id}/invoke to create unified delivery records and emit to Redis Streams
• Remove AddonInvocationWorker and AddonInvocationStore
• Route addon.invoked events through WebhookEventConsumer → unified delivery worker
• Add X-TMI-Callback: async response handling to delivery worker
• Update cleanup worker to handle unified deliveries
• Update OpenAPI spec, tests, docs, wiki

Phase 3: Migrate Resource-Change Deliveries from Postgres to Redis

• Repoint WebhookEventConsumer to create Redis delivery records instead of Postgres
• Repoint admin endpoints (/admin/webhooks/deliveries/*) to read from Redis
• Remove GormWebhookDeliveryStore and webhook_deliveries Postgres table
• Add migration to drop the Postgres table
• Update tests