test: enable dormant DB integration tests against testcontainers

[mrofreP]

integration/database_integration_test.go was written before the
testcontainers-backed integration harness (main_test.go) landed and
was never re-enabled. Every top-level test and benchmark starts with
an unconditional t.Skip("... requires actual database setup"), while
main_test.go already provisions a real Postgres container and exposes
it via the package-level db (*models.Queries) and dbPool.

Wire the tests to the existing harness, drop the unconditional skips,
and delete three pure-placeholder tests whose bodies were just t.Log
("would be implemented here"): TestTransactionIntegrity,
TestConcurrentOperations, TestPerformanceQueries. They were dead
scaffolding, not tests.

Also:

• Remove the unused local queries *models.Queries var and its stub
  setupTestDatabase helper; database/sql import goes with them.
• testUserOperations now creates a per-run unique user with a valid
  username length and all required fields (language_id, question_id,
  verificationdata, signup_ts, last_updated_by), matching the pattern
  used in manager_change_test.go's setupTestChannelAndUser.
• testChannelOperations / testChannelMembershipOperations /
  testComplexQueries keep their defensive t.Skip guards for the
  seeded-data cases, which is the correct behavior when running
  against a clean container.
• BenchmarkChannelSearch and BenchmarkUserChannelMemberships lose
  their unconditional skips but keep the short-mode guard.

Net effect: TestDatabaseIntegration now runs 4 subtests
(User/Channel/Membership/Complex) against the real Postgres, adding
integration coverage to the sqlc-generated models package that
previously had 0% unit coverage.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

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