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/database.py

@@ -56,4 +56,11 @@ def create_db_and_tables() -> None:
             "INSERT INTO logs_fts(logs_fts, rowid, message) "
             "VALUES('delete', old.id, old.message); END"
         )
+        # Idempotent column additions for schema evolution (no Alembic)
+        try:
+            conn.exec_driver_sql(
+                "ALTER TABLE replicas ADD COLUMN promoted_from_master INTEGER DEFAULT 0"
+            )
+        except Exception:
+            pass  # column already exists
         conn.commit()