feat: add push_subscriptions table, VAPID config, and web-push depend…

[Bellamzy]

feat: push subscriptions with VAPID support

Adds the push_subscriptions table and all supporting infrastructure for Web Push notifications.

Changes:

• Schema — push_subscriptions table tied to user_devices.id (cascade delete) with endpoint, p256dh, auth, createdAt,
  lastUsedAt, disabledAt
• Migration — 0009_push_subscriptions.sql creates the table with a unique constraint on endpoint and FK to user_devices
• Dependency — web-push@3.6.7 + @types/web-push@3.6.4 added to backend
• VAPID config — VAPID_PUBLIC_KEY, VAPID_PRIVATE_KEY, VAPID_SUBJECT added to EnvSchema (validated at startup);
  NEXT_PUBLIC_VAPID_PUBLIC_KEY exposed to the browser via next.config.ts
• Push route — POST /push/subscriptions upserts subscription and sets lastUsedAt; re-subscribing clears disabledAt. Exports
  touchSubscription / disableSubscription helpers for use when sending notifications
• Env — .env.example updated with all VAPID variables

Testing:

Generate VAPID keys and populate .env before running:

node -e "const wp = require('web-push'); console.log(wp.generateVAPIDKeys())"
closes #234

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