feat: email unsubscribe tokens with one-click opt-out

- Migration 0019: email_unsubscribe_token column on users (unique, indexed)
- Token auto-generated on first email address save (same pattern as RSS token)
- GET /api/notifications/unsubscribe/{token} — no auth required, sets
  email_enabled=False and returns a branded HTML confirmation page
- List-Unsubscribe + List-Unsubscribe-Post headers on every email
  (improves deliverability; enables one-click unsubscribe in Gmail/Outlook)
- Unsubscribe link appended to email body plain text

Authored by: Jack Levy
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/models/user.py

@@ -15,6 +15,7 @@ class User(Base):
     is_admin = Column(Boolean, nullable=False, default=False)
     notification_prefs = Column(JSONB, nullable=False, default=dict)
     rss_token = Column(String, unique=True, nullable=True, index=True)
+    email_unsubscribe_token = Column(String(64), unique=True, nullable=True, index=True)
     created_at = Column(DateTime(timezone=True), server_default=func.now())
 
     follows = relationship("Follow", back_populates="user", cascade="all, delete-orphan")