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/repositories/import_log_repo.py

@@ -50,12 +50,15 @@ async def add_log(
     Returns:
         Primary key of the inserted row.
     """
+    import time
+
+    timestamp_unix: int = int(time.time())
     cursor = await db.execute(
         """
-        INSERT INTO import_log (source_id, source_url, ips_imported, ips_skipped, errors)
-        VALUES (?, ?, ?, ?, ?)
+        INSERT INTO import_log (source_id, source_url, timestamp, ips_imported, ips_skipped, errors)
+        VALUES (?, ?, ?, ?, ?, ?)
         """,
-        (source_id, source_url, ips_imported, ips_skipped, errors),
+        (source_id, source_url, timestamp_unix, ips_imported, ips_skipped, errors),
     )
     await db.commit()
     return int(cursor.lastrowid)  # type: ignore[arg-type]