Refactor: Add schema versioning and migrations for the SQLite runtime store

[techmore]

Title: Add schema versioning and migrations for the SQLite runtime store

Type: refactor
Severity: high
Area: runtime database / persistence architecture

Description:
The SQLite runtime store now underpins jobs, logs, reports, history, customer history, maintenance status, and export. Despite that central role, the store has no explicit schema versioning or migration framework. It initializes tables opportunistically with CREATE TABLE IF NOT EXISTS, but there is no PRAGMA user_version, migration registry, or startup compatibility check. Future schema changes will be forced into ad hoc conditional SQL and risk silent incompatibilities for installed databases.

Evidence:

• /Users/techmore/projects/NmapUI/nmapui/runtime_db.py defines SCHEMA_STATEMENTS and runs them in initialize() but does not track a runtime DB schema version
• the runtime DB is now used widely across /Users/techmore/projects/NmapUI/nmapui/handlers/routes.py, /Users/techmore/projects/NmapUI/nmapui/handlers/connections.py, and report/history flows, which raises the cost of incompatible schema drift
• migration/export behavior in /Users/techmore/projects/NmapUI/build.sh and /Users/techmore/projects/NmapUI/scripts/backfill_runtime_store.py assumes the DB can be copied forward without explicit version negotiation

Proposed Fix:
Introduce versioned runtime DB migrations with explicit startup enforcement.

Implementation Notes:

• Use PRAGMA user_version or a dedicated metadata table as the single schema version source
• Add ordered migration functions and tests for upgrade paths
• Fail clearly on unsupported future versions instead of implicitly operating on unknown schemas
• Add an admin command to inspect and repair runtime DB versions

Related Issues:

• Architecture: Move runtime state and replay onto a SQLite-backed state store #126
• Enhancement: Add runtime database export and migration build path #131
• Bug: SQLite runtime store is vulnerable to database-locked failures under concurrent Socket.IO/background writes #136

[techmore]

Implemented explicit runtime DB schema versioning on branch codex/sqlite-lock-hardening.\n\nDelivered:\n- PRAGMA user_version-backed schema version tracking\n- ordered migration map via SCHEMA_MIGRATIONS\n- startup migration through RuntimeStateStore.initialize()\n- hard failure when a DB schema is newer than the running app supports\n- regression coverage for current-version initialization, upgrade from legacy version 0, and rejection of unknown future schema versions\n\nVerification:\n- ./.venv/bin/python -m pytest -q tests/test_runtime_db.py tests/test_runtime_contract.py\n- ./.venv/bin/python -m pytest -q\n\nResult: 274 passed, 6 skipped

[techmore]

Closing this because schema versioning and startup migration handling are now implemented on in commit .