Refuse a port the function cannot take, and read a failing node as degraded

A python node's ports and settings arrive as keyword arguments, so a declared
name its `process` does not take was a TypeError on every call — and a node
that loads fine and fails every time it runs is the quiet kind of broken: the
hosted demo did it 720 times an hour for two days and the health badge read
ok throughout. `_build_node` now reads a written body with `ast` and refuses
the mismatch at load, so the node is an error on the canvas and an issue on
publish. Skipped for `**kwargs`, a decorated or absent `process`, and the
template a new node opens with. The SDK's generated shim always takes
`**settings`, so synced flows are untouched.

`/observability/summary` names a node that has failed in the last fifteen
minutes and reads degraded while it does, which is what would have made the
badge amber. `nodes.failing` carries the count.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SkgNtaR6JspnHBFFP6crZj

backend/fluksio/api/routes/observability.py

@@ -7,6 +7,7 @@ which the generated SDK turns into a thrown error — and a health page that
 cannot render while the engine is degraded is the wrong way round.
 """
 
+import time
 from datetime import UTC, datetime, timedelta
 from typing import Annotated, Any, Literal
 
@@ -191,6 +192,20 @@ async def read_summary(request: Request, controller: FlowControllerDep) -> Any:
             f"{len(unhealthy)} node(s) down: "
             f"{', '.join(sorted(e.id for e in unhealthy))}"
         )
+    # A node that loads and then fails on every call is not in `errored`, and
+    # until this it read as healthy. ponytail: one failure reads degraded for
+    # 15 minutes; a counter with decay if that proves noisy.
+    now = time.time()
+    failing = [
+        e
+        for e in entries
+        if e.last_error_ts is not None and now - e.last_error_ts < 900
+    ]
+    if failing:
+        problems.append(
+            f"{len(failing)} node(s) failing: "
+            f"{', '.join(sorted(e.id for e in failing))}"
+        )
 
     # What the canvas flags on a flow — a dependency loop, an input nothing
     # feeds — stops that flow running just as surely as a node that will not
@@ -234,6 +249,7 @@ async def read_summary(request: Request, controller: FlowControllerDep) -> Any:
             "total": len(entries),
             "error": len(errored),
             "unhealthy": len(unhealthy),
+            "failing": len(failing),
         },
         queue=queue,
         loop_lag=(