Add automatic schema versioning with startup migration

On startup, the app now checks if the database schema version matches
the code. If there's a mismatch or no version exists, it automatically
runs a full data migration before starting.

- Add backend/version.py with SCHEMA_VERSION constant
- Add backend/migration.py with extracted migration logic
- Add SchemaVersion model to track DB version
- Add version check functions to database.py
- Update app.py lifespan to use check_and_migrate_if_needed()
- Simplify migrate_csv_to_db.py to use shared logic

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app.py

@@ -26,7 +26,7 @@ from .data_loader import (
     geocode_single_postcode,
 )
 from .data_loader import get_data_info as get_db_info
-from .database import init_db
+from .database import check_and_migrate_if_needed
 from .schemas import METRIC_DEFINITIONS, RANKING_COLUMNS, SCHOOL_COLUMNS
 from .utils import clean_for_json
 
@@ -135,16 +135,16 @@ def validate_postcode(postcode: Optional[str]) -> Optional[str]:
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     """Application lifespan - startup and shutdown events."""
-    # Startup: initialize database and pre-load data
-    print("Starting up: Initializing database...")
-    init_db()  # Ensure tables exist
+    # Startup: check schema version and migrate if needed
+    print("Starting up: Checking database schema...")
+    check_and_migrate_if_needed()
 
     print("Loading school data from database...")
     df = load_school_data()
     if df.empty:
-        print("Warning: No data in database. Run the migration script to import data.")
+        print("Warning: No data in database. Check CSV files in data/ folder.")
     else:
-        print("Data loaded successfully.")
+        print(f"Data loaded successfully: {len(df)} records.")
 
     yield  # Application runs here