feat(dashboard): console phase 2 - self-hosted basemap, basin stations, markers

- Basemap: src/static/basemap/ping-basin.pmtiles, a Protomaps extract of the
  Ping basin (zoom 0-12, 15 MB, OpenStreetMap data), drawn by
  protomaps-leaflet 5.1.0 (unpkg, SRI) in its light/dark style with labels in
  the page language. It replaces the dimmed OSM raster tiles, whose baked-in
  labels were the noisiest part of the screen; OSM stays as the fallback.
  The browser reads the file with range requests (ServeDir answers 206;
  dev_proxy.py now does too). The map stays within the basin.
- Basin stations: a second list next to the RID gauges and a map layer of
  diamonds, from /api/hii/waterlevel/latest (falling back to
  /sensors/thaiwater), coloured by ThaiWater's bank-height classes (over the
  bank = danger, 70-100 % = watch). Gauges RID already lists appear once.
  The panel shows level (m MSL), % of bank height, distance to and height of
  the bank top, gauge reading and agency; selection in the URL (#hii-<id>).
- custom-markers.json as pins from zoom 12, with the flood zone they are in.

Refresh instructions for the basemap in src/static/basemap/README.md.

scripts/dev_proxy.py

@@ -8,6 +8,7 @@ console preview at /static/console.html), everything else the live server."""
 import http.server
 import mimetypes
 import os
+import re
 import sys
 import urllib.request
 from pathlib import Path
@@ -38,7 +39,17 @@ class Handler(http.server.BaseHTTPRequestHandler):
                 )
                 if ctype.startswith("text/"):
                     ctype += "; charset=utf-8"
-                self._send(200, ctype, f.read_bytes())
+                body = f.read_bytes()
+                # Byte ranges, like the server's ServeDir: PMTiles reads the
+                # basemap in pieces
+                m = re.fullmatch(r"bytes=(\d+)-(\d*)", self.headers.get("Range", ""))
+                if m:
+                    start = int(m.group(1))
+                    end = min(int(m.group(2) or len(body) - 1), len(body) - 1)
+                    extra = {"Content-Range": f"bytes {start}-{end}/{len(body)}"}
+                    self._send(206, ctype, body[start : end + 1], extra)
+                    return
+                self._send(200, ctype, body)
                 return
         try:
             req = urllib.request.Request(
@@ -57,9 +68,12 @@ class Handler(http.server.BaseHTTPRequestHandler):
         except urllib.error.HTTPError as e:
             self._send(e.code, "application/json", e.read())
 
-    def _send(self, code, ctype, body):
+    def _send(self, code, ctype, body, extra=None):
         self.send_response(code)
         self.send_header("Content-Type", ctype)
+        self.send_header("Accept-Ranges", "bytes")
+        for name, value in (extra or {}).items():
+            self.send_header(name, value)
         self.send_header("Content-Length", str(len(body)))
         self.send_header("Cache-Control", "no-store")
         self.end_headers()