fix: restore station telemetry and make river flow visualization data-driven

Station selection showed no history since 21ca844: Chart.js v4 datasets
had parsing:false with plain number arrays, drawing empty axes. Remove
the flag so the chart parses values again.

Backend hardening for the same flow:
- /measurements/history/{code} no longer 503s on non-Postgres configs;
  it falls back to the configured adapter (reversed to ascending order)
- DB_TYPE defaults to postgresql when POSTGRES_CONNECTION_STRING is set
  and DB_TYPE is unset, so the .env psql wins over the sqlite default
- zero readings (0.0) are no longer coerced to None, which would fail
  MeasurementResponse validation and 500 /measurements/latest

Map visualization:
- river segments are now colored, widened and dash-speed-animated by
  the discharge at the nearest gauge (same scale as the marker legend)
- fix z-order bug that drew the animated flow line behind its casing
- legend entries for river lines, reduced-motion fallback

River geometry: rebuild ping-river-network.geojson from Overpass
(110 -> 202 features), restoring missing Ping mainstem reaches through
the Bhumibol reservoir and the Tak-Kamphaeng Phet braided section
(unnamed waterway=river ways in OSM), with short synthetic connectors
(connector: true) bridging remaining sub-8 km holes.

src/config.py

@@ -45,7 +45,11 @@ class Config:
     )
 
     # Database configuration
-    DB_TYPE = os.getenv("DB_TYPE", "sqlite").lower()
+    # When DB_TYPE is not set explicitly, a configured Postgres connection wins over the sqlite default
+    DB_TYPE = os.getenv(
+        "DB_TYPE",
+        "postgresql" if os.getenv("POSTGRES_CONNECTION_STRING") else "sqlite",
+    ).lower()
 
     # VictoriaMetrics settings
     # Default to localhost; set VM_HOST in the environment for real deployments