feat(notifications): follow modes, milestone alerts, notification enhancements

Follow Modes (neutral / pocket_veto / pocket_boost):
- Alembic migration 0013 adds follow_mode column to follows table
- FollowButton rewritten as mode-aware dropdown for bills; simple toggle for members/topics
- PATCH /api/follows/{id}/mode endpoint with validation
- Dispatcher filters pocket_veto follows (suppress new_document/new_amendment events)
- Dispatcher adds ntfy Actions header for pocket_boost follows

Change-driven (milestone) Alerts:
- New notification_utils.py with shared emit helpers and 30-min dedup
- congress_poller emits bill_updated events on milestone action text
- llm_processor replaced with shared emit util (also notifies member/topic followers)

Notification Enhancements:
- ntfy priority levels (high for bill_updated, default for others)
- Quiet hours (UTC): dispatcher holds events outside allowed window
- Digest mode (daily/weekly): send_notification_digest Celery beat task
- Notification history endpoint + Recent Alerts UI section
- Enriched following page (bill titles, member photos/details via sub-components)
- Follow mode test buttons in admin settings panel

Infrastructure:
- nginx: switch upstream blocks to set $variable proxy_pass so Docker DNS
  re-resolves upstream IPs after container rebuilds (valid=10s)
- TROUBLESHOOTING.md documenting common Docker/nginx/postgres gotchas

Authored-By: Jack Levy

backend/app/models/follow.py

@@ -12,6 +12,7 @@ class Follow(Base):
     user_id = Column(Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False)
     follow_type = Column(String(20), nullable=False)   # bill | member | topic
     follow_value = Column(String, nullable=False)       # bill_id | bioguide_id | tag string
+    follow_mode = Column(String(20), nullable=False, default="neutral")  # neutral | pocket_veto | pocket_boost
     created_at = Column(DateTime(timezone=True), server_default=func.now())
 
     user = relationship("User", back_populates="follows")