feat(webhooks): persist subscriptions in PostgreSQL

[woahwhattheheck]

Summary

• replace process-local webhook subscription storage with the existing PostgreSQL pool
• add migration 032 for tenant-scoped webhook subscriptions, including organization and event indexes
• make list/delete database-backed and add PATCH /subscriptions/:id so the subscription API is full CRUD
• make dispatch read the shared subscription table so every API replica observes the same subscriptions
• keep list/update/delete explicitly scoped by organization_id
• adapt the existing webhook route tests to the persistent storage contract without adding a parallel test suite

Scope

This is limited to subscription persistence and CRUD. It does not add webhook delivery history, delivery tracking, or a broader dispatch redesign.

Validation

• branch is one commit ahead of current sponsor main (171c74b) and zero commits behind
• reviewed the exact 5-file diff and migration against the repository's existing database/migration conventions
• focused existing route tests were updated for database-backed CRUD and tenant isolation
• the backend test suite was not executed for this submission; the repository's current PR workflow does not run backend tests

Closes #502

[woahwhattheheck]

/claim #502

Existing issue claim: #502 (comment)

Original contributor and claimant/payee: @woahwhattheheck. Please evaluate PR #704 under the issue's existing GrantFox OSS / Third Campaign bounty route. Current carrier head: e74793a8388b091f62618ea95f58dd3d85a64b72.