chore(routes): default packet_count_threshold=5 and clear multiplier=3x

Change the new-route defaults so the create modal, REST API, YAML
import, and preview helper pre-fill packet_count_threshold=5 instead
of 3, and bump the effective-clear auto-multiplier from 2x to 3x so
the default comfort bar tracks to 15 for a default threshold of 5.

Schema/model/cli/preview:
- schemas/routes.py: RouteCreate + RoutePreviewRequest
  packet_count_threshold default 3 -> 5 (clear_threshold stays None)
- models/route.py: Route INSERT default 3 -> 5 (clear_threshold stays
  nullable, no default)
- collector/cli.py: YAML import fallbacks 3 -> 5
- collector/routes.py: preview helper fallback 3 -> 5; module constant
  CLEAR_DEFAULT_MULTIPLIER 2 -> 3 (drives effective_clear_threshold)
- api/metrics.py: route_clear gauge help text 2x -> 3x

UI:
- web/spa/pages/routes.js: new-route modal pre-fills
  packet_count_threshold=5; placeholder shows 3x threshold; parseInt
  fallback 3 -> 5. clear_threshold field unchanged (clearing still
  sends null for the auto-tracking behaviour).

Docs:
- docs/routes.md defaults table: window_hours 24->48, threshold 3->5,
  clear (2x)->(3x), max_hop_span (unlimited)->8. Previous four
  entries now match the shipped defaults (some were stale from #316).
- docs/seeding.md YAML example: window_hours 24->48, threshold 3->5,
  commented clear_threshold example 10->15 with 3x note.

No migration: packet_count_threshold uses Python-side default= (no
server_default), so existing rows keep their stored values. Existing
routes with clear_threshold=NULL continue to track via the new 3x
multiplier at evaluation time.

tests/test_api/test_routes.py

@@ -390,7 +390,7 @@ class TestRouteQualityAvg:
                     quality="clear",
                     matched_count=1,
                     threshold=route.packet_count_threshold,
-                    effective_clear=route.packet_count_threshold * 2,
+                    effective_clear=route.packet_count_threshold * 3,
                     quality_avg=value,
                 )
             )