chore(types): second mypy slice (#158)

[jeffmcutter]

Summary

• Second slice of the mypy burn-down in Make mypy clean and enforce it in pre-commit + CI #158: add missing annotations and narrow JSON/API ids in migration/ (coordinator, importer, exporter, transformer, state, and related modules).
• mypy src/aap_migration/migration is green. Does not add mypy to pre-commit or CI. CLI typing leftovers and Fix CLI/API drift in checkpoint and config show commands #173 (checkpoint / config show API drift) remain for later slices.
• Independent of chore(types): first mypy slice (#158) #174 (first slice); this branch is from main. After both land, remaining mypy src/ failures should be CLI-only.

Test plan

• mypy src/aap_migration/migration is clean
• Pre-commit (ruff/black/unit tests) passed on the commit
• Local migration test completed successfully
• Reviewers: spot-check _as_int / _coerce_source_id guards and the CoordinatorMetrics TypedDict — those are the behavior-adjacent changes

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