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/version.py

@@ -0,0 +1,22 @@
+"""
+Schema versioning for database migrations.
+
+HOW TO USE:
+- Bump SCHEMA_VERSION when making changes to database models
+- This triggers an automatic full data reimport on next app startup
+
+WHEN TO BUMP:
+- Adding/removing columns in models.py
+- Changing column types or constraints
+- Modifying CSV column mappings in schemas.py
+- Any change that requires fresh data import
+"""
+
+# Current schema version - increment when models change
+SCHEMA_VERSION = 2
+
+# Changelog for documentation
+SCHEMA_CHANGELOG = {
+    1: "Initial schema with School and SchoolResult tables",
+    2: "Added pupil absence fields (reading, maths, gps, writing, science)",
+}