Add /api/version capability endpoint for FE/BE version negotiation

Claude-Session: https://claude.ai/code/session_01RdARHDJaYC1wSQRTYM6t3Z

tests/web/test_api.py

@@ -58,6 +58,14 @@ def test_grade_is_aliased_across_api_versions(client):
 def test_query_validation_rejects_out_of_range(client):
     assert client.get("/thermograph/api/v2/grade",
                       params={"lat": 999, "lon": 0}).status_code == 422
+
+
+def test_api_version_reports_backend_contract(client):
+    r = client.get("/thermograph/api/version")
+    assert r.status_code == 200
+    body = r.json()
+    assert {"backend_version", "min_frontend", "payload_ver"} <= body.keys()
+    assert body["backend_version"] == "2"
     assert client.get("/thermograph/api/v2/grade",
                       params={**Q, "days": 0}).status_code == 422
 

web/app.py

@@ -212,6 +212,22 @@ def healthz():
     return {"status": "ok", "role": ROLE}
 
 
+API_CONTRACT_VERSION = "2"   # bump only on a breaking change to any /api/v{N} route or payload shape
+MIN_SUPPORTED_FRONTEND = "1" # oldest frontend contract version this backend still serves correctly
+
+
+@app.get(f"{BASE}/api/version")
+def api_version():
+    """Capability/version-negotiation endpoint: lets a split-out frontend detect at
+    boot whether the backend it's talking to still supports its contract.
+    Deliberately does no I/O, same posture as /healthz."""
+    return {
+        "backend_version": API_CONTRACT_VERSION,
+        "min_frontend": MIN_SUPPORTED_FRONTEND,
+        "payload_ver": payloads.PAYLOAD_VER,
+    }
+
+
 def _client_ip(request) -> "str | None":
     """The real client IP: the left-most X-Forwarded-For hop when a proxy fronts us
     (Caddy on prod sets it), otherwise the direct peer address (LAN dev)."""