feat: add email notification system for bounty updates (#841) [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Summary

Implements the Email Notification System for bounty updates (Bounty #841, T2 Backend, 400K FNDRY).

Contributors can now receive email alerts when bounties matching their interests are posted, updated, or completed, with digest options and delivery tracking / bounce handling.

What's included

Email templates (backend/app/email_templates.py)

• 🔨 New Bounty — dark-themed card with tier badge, reward, and skill tags
• 📋 Status Update — color-coded status change (approved / changes_requested / merged / cancelled)
• 💰 Payout — reward amount display with transaction link
• 📊 Weekly/Daily Digest — aggregated new + completed bounties

Email service (backend/app/email_service.py)

• SendGrid REST API delivery (already uses httpx, an existing backend dependency)
• Falls back to dev-mode logging when SENDGRID_API_KEY is unset
• Supports tracking_id custom args for delivery correlation

User preference management (backend/app/services/preference_store.py)

• Persistent JSON-file-backed store (survives restarts — not an in-memory dict)
• Per-user: contact email, notification frequency (instant/daily/weekly/off), per-type opt-in/opt-out, digest day
• Thread-safe, atomic writes

Notification orchestration (backend/app/services/notify.py)

• notify_bounty_event() — single entry point that the bounty create / update / complete flows call
• Looks up subscribers who opted into each event type and respects their frequency preference
• Returns the number of emails sent

Endpoints (backend/app/api/v1/endpoints/)

• GET /api/notifications/preferences — read a user's preferences
• PUT /api/notifications/preferences — update preferences
• POST /api/notifications/send — test/admin trigger
• POST /api/webhooks/sendgrid/events — SendGrid delivery webhook (bounce handling)

Tests (backend/test_email_notifications.py)

• 15 tests: template rendering, SendGrid success/failure, preference persistence, notification routing — all passing

Acceptance criteria coverage

• ✅ Email templates for bounty updates and status changes
• ✅ User preference management for notification frequency
• ✅ Email delivery tracking and bounce handling (SendGrid event webhook)

Closes #841