fix: break moderation infinite re-review loop by adding 'flagged' status

When AI flags an event, transition status to 'flagged' instead of
leaving it as 'pending'. This prevents processPending() from picking
up the same flagged events every 30-second poll cycle and burning
AI tokens indefinitely.

- Add 'flagged' to QUEUE_STATUSES enum in schema
- Set status='flagged' in autoReview() when AI flags an event
- Include flagged count in admin stats endpoint
- Add index on relay_event_queue.status for efficient queries

Fixes #27

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/db/migrations/0007_moderation_flagged_status.sql

@@ -0,0 +1,9 @@
+-- Migration: Add 'flagged' status to relay_event_queue
+-- Fixes infinite re-review loop (#27): AI-flagged events now transition to
+-- status='flagged' instead of staying 'pending', so processPending() skips them.
+--
+-- The status column is plain TEXT (not a Postgres enum), so no ALTER TYPE is
+-- needed. This migration adds an index for admin queries on flagged events.
+
+CREATE INDEX IF NOT EXISTS idx_relay_event_queue_status
+  ON relay_event_queue(status);