Add SQLite-backed alerts API

[spalen0]

Summary

• add SQLite-backed alert persistence and best-effort capture from Telegram, structured alerts, crash alerts, ops errors, and automation digests
• add stdlib read-only alerts API plus monitoring-api systemd unit, deploy install integration, runbook docs, and retention job
• migrate utils.cache state to SQLite with legacy text read-through, CACHE_BACKEND=file rollback, and deploy/migrate-file-cache-to-db.sh for live imports

Verification

• uv run --extra dev ruff format .
• uv run --extra dev ruff check .
• uv run --extra dev mypy --explicit-package-bases utils/paths.py utils/store.py utils/cache.py utils/migrate_cache_to_db.py utils/telegram.py utils/alert.py utils/runner.py automation/runner.py api utils/prune_alerts.py
• uv run --extra dev pytest tests/ (502 passed, 4 skipped)
• bash -n deploy/install.sh
• bash -n deploy/migrate-file-cache-to-db.sh
• deploy migration wrapper smoke-tested against a temp CACHE_DIR