Add durable delivery database core

[dakdevs]

What changed

• adds the Drizzle schema and initial PostgreSQL migration for accounts, hooks, immutable webhook events, deliveries, and delivery attempts
• adds a tenant-scoped DeliveryStore for atomic event recording, concurrent claims, ACK/NACK, heartbeat extension, and lease recovery
• stores only hashes of short-lived lease tokens
• adds a real-PostgreSQL integration harness that launches an ephemeral local cluster
• adds a PostgreSQL 16 service to CI and documents Neon pooled connections and migration commands

Why

This implements the durable database core from PRD #1 before the Vercel ingress and CLI are wired to it. PostgreSQL is the source of truth, and all correctness-sensitive transitions use short atomic statements.

This is stacked on #2 and should be reviewed after the bootstrap PR.

Refs #1
Depends on #2

Validation

• bun run db:generate reports no schema drift
• bun run lint
• bun run prettier:check
• bun run typecheck
• bun test — 11 tests, including 10 real-PostgreSQL delivery tests
• bun run test:e2e
• bun run build

The integration suite covers concurrent claim exclusion, tenant isolation, lease expiry and redelivery, stale-token rejection, retry scheduling, heartbeat extension, disabled hooks, and database-enforced webhook-event immutability.

[dakdevs]

Superseded by #8, which is merged into main.