Add Economic Observatory web UI with ledger-backed API

Introduce ui/ dashboard (dark observatory layout), JSON API, and local
dev server. All metrics load from expense and payment record ledgers.
Links Claude design reference for visual alignment.

projects/coulomb-pricing/observatory/server.py

@@ -0,0 +1,72 @@
+from __future__ import annotations
+
+import argparse
+import json
+from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
+from pathlib import Path
+from urllib.parse import parse_qs, urlparse
+
+from .api import build_dashboard_payload
+
+ROOT = Path(__file__).resolve().parent.parent
+UI_DIR = ROOT / "ui"
+
+
+class ObservatoryHandler(BaseHTTPRequestHandler):
+    data_dir: Path = ROOT / "data"
+
+    def _send(self, status: int, body: bytes, content_type: str) -> None:
+        self.send_response(status)
+        self.send_header("Content-Type", content_type)
+        self.send_header("Access-Control-Allow-Origin", "*")
+        self.end_headers()
+        self.wfile.write(body)
+
+    def do_GET(self) -> None:
+        parsed = urlparse(self.path)
+        if parsed.path == "/api/dashboard":
+            query = parse_qs(parsed.query)
+            period = query.get("period", [None])[0]
+            payload = build_dashboard_payload(self.data_dir, period)
+            self._send(200, json.dumps(payload).encode("utf-8"), "application/json")
+            return
+
+        if parsed.path == "/":
+            return self._serve_file(UI_DIR / "index.html", "text/html; charset=utf-8")
+
+        if parsed.path.startswith("/ui/"):
+            relative = parsed.path.removeprefix("/ui/")
+            target = UI_DIR / relative
+            if target.exists() and target.is_file():
+                content_type = "text/css" if target.suffix == ".css" else "application/javascript"
+                return self._serve_file(target, f"{content_type}; charset=utf-8")
+
+        self._send(404, b"Not found", "text/plain")
+
+    def _serve_file(self, path: Path, content_type: str) -> None:
+        self._send(200, path.read_bytes(), content_type)
+
+    def log_message(self, format: str, *args) -> None:
+        return
+
+
+def main(argv: list[str] | None = None) -> int:
+    parser = argparse.ArgumentParser(description="Coulomb Economic Observatory UI server")
+    parser.add_argument("--host", default="127.0.0.1")
+    parser.add_argument("--port", type=int, default=8765)
+    parser.add_argument("--data-dir", type=Path, default=ROOT / "data")
+    args = parser.parse_args(argv)
+
+    ObservatoryHandler.data_dir = args.data_dir
+    server = ThreadingHTTPServer((args.host, args.port), ObservatoryHandler)
+    print(f"Economic Observatory UI: http://{args.host}:{args.port}/")
+    print(f"API: http://{args.host}:{args.port}/api/dashboard")
+    try:
+        server.serve_forever()
+    except KeyboardInterrupt:
+        print("\nStopped.")
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())