fix: invalidate gateway cache on secret and rule mutations

[guyb1]

Summary

Credentials added via the dashboard were not immediately available to agents due to the gateway's 60-second CONNECT response cache. This adds cache invalidation so changes take effect instantly.

Changes

Gateway (Rust):

• Add del_by_prefix method to CacheStore trait (both InMemory and Redis implementations)
• Include account_id in cache key format: connect:{account_id}:{token}:{host}
• Add POST /api/cache/invalidate endpoint (authenticated, scoped to caller's account)

Web app (TypeScript):

• Add useInvalidateGatewayCache hook for client-side cache busting
• Call it after every secret create/update/delete, rule create/update/delete/toggle, agent token regeneration, agent deletion, and agent secret assignment changes

How it works

1. User mutates a secret/rule/agent in the dashboard
2. Server action saves to DB
3. Client fires POST /api/cache/invalidate to the gateway (fire-and-forget)
4. Gateway clears all cached CONNECT responses for that account
5. Next agent request hits the DB and gets fresh data

Fixes #116