Add DB schema regression tests

[Kismettum]

Closes #672.

Adds an instrumentation regression test for database schema drift.

The test covers:

• Fresh installs by opening a dedicated test database through SQLiteOpenHelper and checking that each current table contains every column used by its table adapter.
• Supported upgrades by creating a frozen v4.20 / DB_VERSION 41 schema, opening it through the current Db upgrade path, and checking the same contracts afterwards.

The expected columns are derived from the existing table adapters' toContentValues() implementations rather than maintaining a separate copy of the current schema.

The test uses isolated temporary databases and mocks isFreshInstall() so it does not modify the app database or create notification channels.

The v4.20 fixture is intended to remain unchanged while that release is inside the supported upgrade window, then be replaced by a newer release baseline when the compatibility window advances.

No production-code changes or new dependencies.

Validation performed before opening this PR:

• reviewed against current master
• static Kotlin/API-shape compilation against matching stubs
• independent SQLite simulation of the v4.20 → current schema path

I have not run the Android instrumentation test on a real/emulated Android device in my environment.

[aj3423]

Looks nice. Thanks.