fix(startup): stop re-migrating on every container restart

Two issues caused the backend to drop and reimport school data on restart:

1. schema_version table was in the drop list inside run_full_migration(),
   so after any migration the breadcrumb was destroyed and the next
   restart would see no version → re-trigger migration
2. Schema version was set after migration, so a crash mid-migration
   left no version → infinite re-migration loop

Fix: remove schema_version from the drop list, and set the version
before running migration so crashes don't cause loops.

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

backend/migration.py

@@ -431,7 +431,8 @@ def run_full_migration(geocode: bool = False) -> bool:
 
     # Only drop the core KS2 tables — leave supplementary tables (ofsted, census,
     # finance, etc.) intact so a reimport doesn't wipe integrator-populated data.
-    ks2_tables = ["school_results", "schools", "schema_version"]
+    # schema_version is NOT dropped: it persists so restarts don't re-trigger migration.
+    ks2_tables = ["school_results", "schools"]
     print(f"Dropping core tables: {ks2_tables} ...")
     inspector = __import__("sqlalchemy").inspect(engine)
     existing = set(inspector.get_table_names())