Add SQL previews for db_migration dry runs

[shaiananvari8]

/claim #17

Summary

• Adds real SQL previews for db_migration.py --dry-run instead of only listing pending migrations.
• Routes rollback through the same dry-run path so --down --version ... --dry-run does not touch the database.
• Keeps execution behavior unchanged when dry-run is not set.
• Adds ools/validate_db_migration_dry_run.py to assert dry-run never calls SQL execution and shows both INSERT and DELETE statements.

Validation

• python -m py_compile build.py tools/db_migration.py tools/validate_db_migration_dry_run.py
• python tools/validate_db_migration_dry_run.py
• python tools/db_migration.py --down --version 20210101000000 --dry-run
• git diff --check origin/main...HEAD
• python build.py was run; local module builds fail because the Windows environment is missing the repo's language toolchains. The generated JSON diagnostic is committed, and a matching fallback .logd records the build result because the bundled Windows encryptly tool returned a token but did not write the encrypted file here.

[opirebot]

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