feat: comprehensive health check with DB, scheduler, cache

- Add /api/v1/health endpoint with component-level checks
- Verify DB connectivity, fail2ban socket, scheduler, session cache
- Add SQLite WAL cleanup on startup (orphan crash files)
- Migration 8: import_log.timestamp → INTEGER UNIX epoch
- Align import_log timestamps with history_archive (already UNIX int)
- Add unit tests for DB cleanup and health router

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/models/blocklist.py

@@ -64,7 +64,7 @@ class ImportLogEntry(BanGuiBaseModel):
     id: int
     source_id: int | None
     source_url: str
-    timestamp: str
+    timestamp: int
     ips_imported: int
     ips_skipped: int
     errors: str | None