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/ui/routes.py

@@ -86,7 +86,7 @@ async def doc_counts_fragment(request: Request, session: Session = Depends(get_s
 
 
 @router.get("/", response_class=HTMLResponse)
-def dashboard(request: Request, session: Session = Depends(get_session)):
+def dashboard(request: Request, session: Session = Depends(get_session), flash: Optional[str] = None):
     replicas = session.exec(select(Replica)).all()
     now = datetime.now(timezone.utc)
     progress = get_progress()
@@ -133,6 +133,7 @@ def dashboard(request: Request, session: Session = Depends(get_session)):
             "replica_rows": replica_rows,
             "last_run": last_run,
             "progress": progress,
+            "flash": flash,
         },
     )