feat: route health monitoring with visual path builder

Add complete route health monitoring feature that tracks whether packets
traverse expected multi-hop paths through the mesh network.

Models & migration:
- 5 new models: PacketPathHop, Route, RouteNode, RouteObserver, RouteResult
- Alembic migration with keyset-paginated backfill of existing packets

Collector:
- store_raw_packet refactored to persist path hops via bulk insert
- Matching engine (collector/routes.py) with subsequence matching, quality
  bands (clear/marginal/failing/no_coverage), and collision detection
- Background route evaluator (60s loop) wired into subscriber lifespan
- Route seed loader in CLI (resolves by public_key, matching YAML format)

API:
- 6 CRUD endpoints + preview endpoint under /api/v1/routes
- Schemas accept node_public_keys (64-char hex) instead of internal UUIDs
- 5 Prometheus gauges for route health metrics
- Packet groups endpoint reads from hop table

Web UI:
- Full SPA routes page with summary strip, grouped cards, expandable detail
- Routes nav entry in both desktop (spa.html) and mobile (app.js) navbars
- Home page nav card with feature gate
- API proxy access mapping for v1/routes endpoints
- Visual node-search path builder with autocomplete dropdown, ordered chips
  with reorder/remove controls, and paste-64-char-key support
- Observer picker with same search UX (unordered chips)
- i18n strings in en.json and nl.json

Config:
- feature_routes flag (default: true)
- route_evaluator_interval_seconds (default: 60)
- routes_file seed path
- example/seed/routes.yaml

.env.example

@@ -603,6 +603,10 @@ SYSTEM_MAINTENANCE=false
 # (SPAM_DETECTION_ENABLED=${FEATURE_SPAM_DETECTION}). Set to false to opt out.
 # See the Spam Detection section above for the scoring tuning vars.
 # FEATURE_SPAM_DETECTION=true
+# Routes page (route health monitoring) is ON by default.
+# FEATURE_ROUTES=true
+# Route evaluator interval in seconds (0 disables, default 60).
+# ROUTE_EVALUATOR_INTERVAL_SECONDS=60
 
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