Add runtime DB schema migration and version tracking

Docs/Tasks.md

@@ -99,6 +99,7 @@ Reference: `Docs/Refactoring.md` for full analysis of each issue.
    - Issue: `app/db.py` only creates missing tables on startup and has no schema version marker or migration strategy. This leaves upgrades brittle and makes it difficult to evolve the database without manual intervention.
    - Propose: Add a lightweight migration system or schema version table, define incremental migration scripts, and run upgrades during startup before the application begins serving requests.
    - Test: Add tests that simulate an older schema version and verify startup migrates it to the latest schema, and assert that `init_db`/migration code leaves the database in the expected current state.
+   - Status: completed
 
 14. Split the monolithic config router into focused sub-routers
    - Goal: Improve maintainability, reduce cognitive load, and make HTTP routing responsibilities easier to isolate and test.

backend/app/db.py

@@ -89,6 +89,13 @@ CREATE TABLE IF NOT EXISTS history_archive (
 );
 """
 
+_CREATE_SCHEMA_MIGRATIONS: str = """
+CREATE TABLE IF NOT EXISTS schema_migrations (
+    version     INTEGER PRIMARY KEY,
+    migrated_at TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ', 'now'))
+);
+"""
+
 # Ordered list of DDL statements to execute on initialisation.
 _SCHEMA_STATEMENTS: list[str] = [
     _CREATE_SETTINGS,
@@ -100,6 +107,12 @@ _SCHEMA_STATEMENTS: list[str] = [
     _CREATE_HISTORY_ARCHIVE,
 ]
 
+_CURRENT_SCHEMA_VERSION: int = 1
+
+_MIGRATIONS: dict[int, str] = {
+    1: "\n".join(_SCHEMA_STATEMENTS),
+}
+
 
 # ---------------------------------------------------------------------------
 # Public API
@@ -113,8 +126,44 @@ async def _configure_connection(db: aiosqlite.Connection) -> None:
     await db.execute("PRAGMA busy_timeout=5000;")
 
 
+async def _get_current_schema_version(db: aiosqlite.Connection) -> int:
+    """Return the highest applied schema version for the given database."""
+    await db.executescript(_CREATE_SCHEMA_MIGRATIONS)
+    async with db.execute("SELECT MAX(version) FROM schema_migrations;") as cursor:
+        row = await cursor.fetchone()
+    if row is None or row[0] is None:
+        return 0
+    return int(row[0])
+
+
+async def _apply_migration(db: aiosqlite.Connection, version: int) -> None:
+    """Apply a single migration step and record its completion."""
+    migration_script = _MIGRATIONS[version]
+    await db.executescript(migration_script)
+    await db.execute("INSERT INTO schema_migrations (version) VALUES (?);", (version,))
+    await db.commit()
+
+
+async def _migrate_schema(db: aiosqlite.Connection) -> None:
+    """Migrate the database schema to the latest supported version."""
+    current_version = await _get_current_schema_version(db)
+    if current_version == _CURRENT_SCHEMA_VERSION:
+        return
+
+    if current_version > _CURRENT_SCHEMA_VERSION:
+        raise RuntimeError(
+            f"database schema version {current_version} is newer than supported "
+            f"version {_CURRENT_SCHEMA_VERSION}"
+        )
+
+    log.info("migrating_database_schema", from_version=current_version, to_version=_CURRENT_SCHEMA_VERSION)
+    for next_version in range(current_version + 1, _CURRENT_SCHEMA_VERSION + 1):
+        await _apply_migration(db, next_version)
+    log.info("database_schema_ready", schema_version=_CURRENT_SCHEMA_VERSION)
+
+
 async def init_db(db: aiosqlite.Connection) -> None:
-    """Create all BanGUI application tables if they do not already exist.
+    """Create or migrate the BanGUI application database schema.
 
     This function is idempotent — calling it on an already-initialised
     database has no effect.  It should be called once during application
@@ -125,10 +174,7 @@ async def init_db(db: aiosqlite.Connection) -> None:
     """
     log.info("initialising_database_schema")
     await _configure_connection(db)
-    for statement in _SCHEMA_STATEMENTS:
-        await db.executescript(statement)
-    await db.commit()
-    log.info("database_schema_ready")
+    await _migrate_schema(db)
 
 
 async def open_db(database_path: str) -> aiosqlite.Connection:

backend/tests/test_repositories/test_db_init.py

@@ -67,3 +67,34 @@ async def test_init_db_is_idempotent(tmp_path: Path) -> None:
     async with aiosqlite.connect(db_path) as db:
         await init_db(db)
         await init_db(db)  # Second call must be a no-op.
+
+
+@pytest.mark.asyncio
+async def test_init_db_records_schema_version(tmp_path: Path) -> None:
+    """``init_db`` must record the current schema version."""
+    db_path = str(tmp_path / "test.db")
+    async with aiosqlite.connect(db_path) as db:
+        await init_db(db)
+        async with db.execute(
+            "SELECT version FROM schema_migrations ORDER BY version DESC LIMIT 1;"
+        ) as cursor:
+            row = await cursor.fetchone()
+    assert row is not None
+    assert row[0] == 1
+
+
+@pytest.mark.asyncio
+async def test_init_db_migrates_legacy_database_without_schema_version(tmp_path: Path) -> None:
+    """Legacy databases without a schema marker must be migrated on startup."""
+    db_path = str(tmp_path / "test.db")
+    async with aiosqlite.connect(db_path) as db:
+        await init_db(db)
+        await db.execute("DROP TABLE schema_migrations;")
+        await db.commit()
+        await init_db(db)
+        async with db.execute(
+            "SELECT version FROM schema_migrations ORDER BY version DESC LIMIT 1;"
+        ) as cursor:
+            row = await cursor.fetchone()
+    assert row is not None
+    assert row[0] == 1