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/tests/test_api.py

@@ -0,0 +1,25 @@
+from __future__ import annotations
+
+import json
+from decimal import Decimal
+from pathlib import Path
+
+from observatory.api import build_dashboard_payload, payload_json
+
+DATA_DIR = Path(__file__).resolve().parent.parent / "data"
+
+
+def test_dashboard_payload_contains_live_ledger_totals() -> None:
+    payload = build_dashboard_payload(DATA_DIR, "2026-06")
+
+    assert payload["period"] == "2026-06"
+    assert payload["liquidity"]["remaining_budget"] == "659.12"
+    assert payload["liquidity"]["cumulative_infrastructure_cost"] == "409.28"
+    assert payload["snapshot"]["monthly_infrastructure_cost"] == "29.73"
+    assert len(payload["history"]) == 18
+    assert payload["expense_record_count"] == 58
+
+
+def test_payload_json_is_valid() -> None:
+    parsed = json.loads(payload_json(DATA_DIR, "2026-06"))
+    assert Decimal(parsed["payments"][0]["fees_amount"]) == Decimal("0.44")