Implement master promotion feature

Allows promoting any replica to master with zero document re-downloads.
The sync_map rebuild uses existing DB data only — pure in-memory join.

Changes:
- app/sync/promote.py: preflight() checks (doc count, sync lock, ack
  warnings) and promote() transaction (pause scheduler, rebuild all
  sync_maps, create old-master replica, swap settings, resume scheduler)
- app/api/master.py: GET /api/master/promote/{id}/preflight (dry run)
  and POST /api/master/promote/{id} (execute)
- app/models.py: add promoted_from_master bool field to Replica
- app/database.py: idempotent ALTER TABLE migration for new column
- app/main.py: register master router
- app/templates/replica_detail.html: "Promote to Master" button +
  dialog with pre-flight summary, 3-card stats, ack checkboxes, spinner
- app/ui/routes.py: flash query param on dashboard route
- app/templates/dashboard.html: blue info banner for post-promotion flash

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

app/templates/dashboard.html

@@ -15,6 +15,12 @@
   </div>
 </div>
 
+{% if flash %}
+<div role="alert" style="padding:0.6rem 1rem; margin-bottom:1rem; background:#eff6ff; color:#1d4ed8; border-radius:0.4rem; border-left:3px solid #1d4ed8; font-size:0.95em;">
+  {{ flash }}
+</div>
+{% endif %}
+
 <!-- Progress bar -->
 <div id="progress-bar" class="{% if progress.running %}active{% endif %}"
      hx-get="/api/sync/running"