feat(tours): route paddle tours on the water, shaping points and auto-loaded regions

Paddle tours are planned on the water by TREK's own router: a water network
per paddle region, built from the same Geofabrik extract as the obstacles and
searched in a worker thread of the server (no sidecar). Rivers, canals and
lakes (direct or along the shore), portages round weirs, locks by policy,
whitewater by boat, gaps marked. Found portages and locks become points of
their own; an unplannable leg says why and offers a way out.

- Shaping points: a click on the line sets a point that steers the route
  without being a stop (hikes and paddle tours); recompute a single leg.
- Lake style direct/shore, suggested from the wind forecast.
- Regions load by themselves where a paddler plans (admin setting: automatic,
  on request, off; size cap and a daily count), with requests for the admins,
  a notification, and the load followed on the leg.
- Portage from the last point, portages follow footways via the walking
  router (or stay straight), "put in here" on slipways and put-ins.
- MCP plan_paddle_route with the REST route's rules.

client/src/styles/paddle.css

@@ -7,12 +7,16 @@
 
 /* Obstacles in slate, access rules in indigo, places to get in and out in
    teal, a carried stretch in earth brown; each marker a disc in the map's own
-   white, which keeps it readable over a satellite tile. */
+   white, which keeps it readable over a satellite tile. A routed paddle tour
+   (PAD-11) marks what the paddler should look at: whitewater and boat passes in
+   amber, a gap in the data or a stretch no route was found for in red. */
 :root {
   --map-paddle-obstacle: #334155;
   --map-paddle-access: #4338ca;
   --map-paddle-landing: #0f766e;
   --map-portage: #9a6233;
+  --map-paddle-rapids: #d97706;
+  --map-paddle-gap: #dc2626;
   --map-paddle-disc: #ffffff;
   --map-paddle-shadow: rgba(15, 23, 42, 0.35);
 }
@@ -22,6 +26,8 @@
   --map-paddle-access: #a5b4fc;
   --map-paddle-landing: #5eead4;
   --map-portage: #d6a06e;
+  --map-paddle-rapids: #f59e0b;
+  --map-paddle-gap: #ef4444;
   --map-paddle-disc: #1e293b;
   --map-paddle-shadow: rgba(0, 0, 0, 0.55);
 }