fix: replace all :: type casts with CAST() in SQLAlchemy text() calls

SQLAlchemy's text() interprets :name::type as two named parameters.
Fixes syntax errors in link discovery, signal history, and SNMP profile
CRUD that caused 500 errors at runtime.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/signal_history_service.py

@@ -51,7 +51,7 @@ async def get_signal_history(
             WHERE wr.mac_address = :mac_address
               AND wr.device_id = :device_id
               AND wr.tenant_id = :tenant_id
-              AND wr.time > now() - :lookback::interval
+              AND wr.time > now() - CAST(:lookback AS interval)
               AND wr.signal_strength IS NOT NULL
             GROUP BY bucket
             ORDER BY bucket