fix(android): register v2→v3 and v3→v4 Room migrations, bump DB to v4

[Hidden-Node]

Registers real Room migrations for the two schema gaps that crash existing
users upgrading to this build, and bumps the DB version to 4 so Room has a
version delta to act on.

Why:

• v2→v3: v1.0.1 bumped version 2→3 without a migration and relied on
  fallbackToDestructiveMigration (removed in plan 003). Any user still on a
  v2 DB now hits "A migration from 2 to 3 was required but not found".
• Within-v3: all v1.0.1–v1.0.6 builds are version 3, but the schema drifted
  (coalesceStepMs, idleSlotsPerBucket, remoteUrl added without a version
  bump), so old v3 DBs crash on "no such column" for every SELECT *.

Changes:

• ProfileMigrations.ALL = [Migration(2,3), Migration(3,4)]; both use an
  existingColumns() guard so one Migration(3,4) handles both live v3 shapes
  (13- and 15-column) and stays idempotent.
• AppDatabase version 3 → 4.
• Commit Room schema JSONs for v2/v3/v4 (enables AutoMigration for future
  column adds).
• Add sqlite-jdbc test dep + ProfileMigrationsTest (5 JVM cases: registration
  wiring, v2→v4 data preservation, both v3 variants, idempotency).

Tests: ./gradlew :app:testDebugUnitTest → BUILD SUCCESSFUL, 5/5 pass.