feat: list pending data migrations, run one by version, and mark repeatable ones

[tw00]

Moves what tv-labs/platform#10396 built inside Sauron into the library, per review there, and adds the one-shot/repeatable annotation.

Summary

• DataMigration.pending(repo, paths) lists the data migrations that have not run, oldest first, as {version, name}. It reads schema_migrations without the migration lock and without creating the table.
• DataMigration.run(repo, version, paths, opts \\ []) runs that one data migration and no other. It runs nothing and returns {:error, :not_found} for a version no file has, {:error, :duplicate_version} for one two files share, and {:error, :already_applied} for a one-shot data migration that has run. opts go to Ecto.Migrator.up/4.
• use DataMigration, repeatable: true in place of use Ecto.Migration marks a data migration run/4 may run again. Anything else, including a plain use Ecto.Migration, is one-shot. Either kind is pending until it has run once. A rerun deletes the version's schema_migrations row (the same delete Ecto.Migrator.down/4 makes, without running down/0), then runs up/0. If that raises, the row stays deleted and the migration shows as pending.
• Test harness: test_helper.exs puts the sandbox in manual mode, and ConnCase stops its owner. In the default auto mode, a process a test starts (a LiveView, the migrator's task) took a connection of its own, and its writes committed. The Postgres, MySQL and MSSQL test repos now set migration_lock: false, as the SQLite one already did: in manual mode the lock holds the test's connection while the migrator's task waits for it. The run/4 tests pass skip_table_creation: true, since MySQL commits the open transaction on the migrator's CREATE TABLE IF NOT EXISTS.
• Version 0.2.0, CHANGELOG under unreleased, README section. Not published: ./bin/release 0.1.2 0.2.0 after merge fills in the date.

The dashboard's "Migrate up" still runs a data migration once. Rerunning a repeatable one from the page can come later.

Evidence

• mix test --warnings-as-errors (Elixir 1.18.4, OTP 27): 32 tests, 0 failures (22 before, 10 new) on SQLite and on Postgres (a local server on another port), each on several seeds. CI runs all four adapters and dialyzer: green on 727aba7.
• The leak: after one mix test on main, the local test.db keeps the page test's 99999999999999 row in schema_migrations. On this branch it has 0 rows after a run, on SQLite and Postgres.
• mix compile --warnings-as-errors, mix format --check-formatted and mix dialyzer (0 errors) are clean. On Elixir 1.20 the only warnings are the existing HEEx ones in page.ex:86 and page.ex:220.
• Sauron (tv-labs/platform#10396) compiled against this lib/data_migration.ex with its three repeatable shims marked: its data migration, Release and admin overview tests pass, 248/248.

🤖 Generated with Claude Code

[tw00]

or 0.1.3?