Tests: add tests for internal/db Migrate

[koinsaari]

internal/db has no tests. Migrate and the PostGIS spatial index creation are exercised only as a side-effect of other integration tests — there is no explicit test that they work on a fresh DB or that re-running them is idempotent.

Suggested coverage

• Spin up a clean Postgres+PostGIS via testcontainers.
• Run Migrate twice; assert no error on the second run (idempotency).
• Assert that the expected tables (places, accessibility_profiles) and the spatial GIST index exist.
• Once Replace AutoMigrate with a real migration tool #9 (real migrations) lands, replace this with proper migration up/down tests.
• File: new internal/db/integration_test.go.

Depends on / pairs with: #9 (replace AutoMigrate with a real migration tool).