feat(webhooks): outbound webhook outbox with dispatcher, retries & admin ops

[priscaenoch]

Summary

Implements the transactional outbox described in #54: durable WebhookEvent rows, a dispatcher with exponential backoff, admin inspection, and payload redaction — preparing partner webhooks without a real HTTP delivery client.

closes #54

• New webhooks module: WebhookEvent entity + migration (indexed on status+nextAttemptAt), WebhooksService (emit/dispatch/retry/listEvents), admin routes under /api/v1/admin/webhook-events (list, dispatch, retry — admin RBAC only, 403 for non-admins), and a deep payload redactor stripping passwords/tokens/secrets/emails from nested objects/arrays.
• Wired emitters: subscription.created on subscribe (both new subscriptions and reactivating a cancelled one) and post.published on post creation — both now write via dataSource.transaction() so the outbox row commits atomically with the domain write.
• No cron/scheduler dependency was installed (none exists in this repo); the dispatcher is triggered via POST /api/v1/admin/webhook-events/dispatch, which is also how tests and admins run it on demand. It logs to console by default, or POSTs to WEBHOOK_DEBUG_URL if configured. Failures back off exponentially (capped at 5 min) and go dead after 5 attempts; dead events can be retried via POST .../:id/retry.
• docs/webhooks-outbox.md documents the data model, emitters, redaction rules, dispatcher behavior, and admin API, including what's intentionally out of scope (signed partner delivery, per-creator webhook management UI).
• 20 new tests in src/webhooks/ (service, redaction, admin controller) plus emitter-specific tests added to the existing subscriptions.service.spec.ts and posts.service.spec.ts.

Pre-existing baseline breakage (unrelated to #54, fixed to keep CI green)

main currently fails npm run build/test:e2e, apparently from a bad merge of a post-soft-delete-archive-restore feature where the controller/tests/migration landed but the service implementation didn't:

• Duplicate AuditAction.USER_SELF_DELETED enum member (build-breaking).
• AuditAction missing several BILLING_WEBHOOK_* members that billing-webhooks.service.ts already referenced.
• FeedService's post mapping was missing the now-required deletedAt field.
• PostsService was missing getArchivedPosts/restorePost (and Post was missing deletedById) even though posts.controller.ts, posts.service.spec.ts, and test/posts.e2e-spec.ts already called them — reconstructed faithfully against those existing call sites and the already-present AddPostSoftDelete migration.
• One e2e assertion (test/posts.e2e-spec.ts) used a numeric user id where every other public-route assertion in that file uses the creator's handle — fixed to match the surrounding pattern.
• 3 files with pre-existing Prettier violations, reformatted.

I confirmed with the requester before making these fixes, since they're outside #54's scope but were blocking build/test/format:check regardless of this PR's contents.

Testing / validation performed

• npm run build — clean.
• npm run format:check — clean.
• npm test — 49 suites / 380 tests pass.
• npm run test:e2e (against a local Postgres via Docker, since no DB is available by default) — test/posts.e2e-spec.ts (43 tests) passes in full, including the reconstructed archive/restore endpoints. Did not run the entire e2e/integration suite end-to-end locally; CI's Postgres service should cover the rest.