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/workers/congress_poller.py

@@ -335,6 +335,15 @@ def _update_bill_if_changed(db, existing: Bill, parsed: dict) -> bool:
         from app.workers.document_fetcher import fetch_bill_documents
         fetch_bill_documents.delay(existing.bill_id)
         fetch_bill_actions.delay(existing.bill_id)
+        from app.workers.notification_utils import (
+            emit_bill_notification,
+            emit_member_follow_notifications,
+            is_milestone_action,
+        )
+        if is_milestone_action(parsed.get("latest_action_text", "")):
+            action_text = parsed["latest_action_text"]
+            emit_bill_notification(db, existing, "bill_updated", action_text)
+            emit_member_follow_notifications(db, existing, "bill_updated", action_text)
     return changed