Phase 0D: Schema-versioned migrations with PRAGMA user_version, backup, integrity check

[hardcoreerik]

Summary

• Replaces ad-hoc _migrate() with a sequential _MIGRATIONS list of (version, fn) pairs — new migrations append without renumbering existing entries
• apply_schema() now reads PRAGMA user_version, runs only migrations with version > current, and stamps the new version inside the same transaction as each migration's ALTER TABLEs
• Pre-migration WAL checkpoint + shutil.copy2 backup to backups/ subfolder before touching any existing database; no backup for brand-new or in-memory connections
• PRAGMA integrity_check runs after all migrations succeed; failure raises MigrationError
• MigrationError(RuntimeError) raised (not swallowed) on any migration or integrity failure — message always includes what failed and where the backup is
• apply_schema() accepts optional db_path for backup; both MonitorStore call sites updated to pass self._path

Test plan

• test_migrates_v0_schema_and_preserves_existing_rows — full v0→current migration, all columns present, pre-existing rows intact
• test_migrates_v1_schema_adding_only_node_columns — skips already-applied v1 message migration, applies only v2 node columns
• test_creates_pre_migration_backup_for_existing_database — backup file appears under backups/ with correct stem pattern
• test_idempotent_reapply_is_a_no_op — second apply_schema() on already-current DB produces no additional backup
• test_fresh_database_reaches_current_version_without_a_backup — new DB stamped at CURRENT_SCHEMA_VERSION, no backups/ directory created
• test_db_path_is_optional — in-memory connection with no db_path does not raise
• test_migration_error_does_not_lose_existing_data — simulated migration failure leaves user_version=0 and pre-existing row untouched
• test_integrity_check_failure_raises_clear_error — fake PRAGMA integrity_check returning non-"ok" raises MigrationError

Full suite: 369 passed, ruff clean, mypy clean.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 3e847ff and 4986fcb.

📒 Files selected for processing (3)

• src/meshchat/database/schema.py
• src/meshchat/services/monitor_store.py
• tests/test_schema_migrations.py

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