feat(webui): read-only system-health API (Closes #634)

Adds `GET /api/v1/system/health`, a structured read-only health surface for
automated readiness checks, and keeps `/health` as the cheap liveness probe.

webui/system_health.py composes a DTO from fail-soft dependency probes: the
control-plane database, the local checkout, and — opt-in via `?deep=1` — live
Gitea reachability, each carrying status, reason, and probe latency. Required
probes drive readiness; the optional Gitea probe can only degrade overall
status, because local inventory stays serveable when the remote is
unreachable. A probe that did not run leaves readiness incomplete rather than
silently passing.

Read-only throughout: the control-plane database is opened through a `mode=ro`
URI because `ControlPlaneDB.__init__` creates directories and runs migrations,
which a health check must never do. No restart or reload control is exposed;
those are Phase 2 and #630 forbids process-kill recovery.

No unproven claims: `stale_runtime.mutation_safe` is true only when the
runtime, checkout, and remote commits are all known and equal, and MCP
namespaces always report `unproven` because a web process cannot exercise the
IDE-managed client path (#543). Probe details are redacted at the browser
boundary — URLs lose userinfo and query strings, credential-shaped text is
masked.

`/health` is expanded additively: every MVP key is retained, plus `started_at`,
`uptime_seconds`, and a pointer to the versioned API. The versioned route
returns 503 when not ready so automation can branch on the status code alone.

Verified at master 9eb0f29: focused file 40 passed / 11 subtests; `-k "webui or
health"` 230 passed / 159 subtests; full suite 4358 passed with the 11
pre-existing master-drift failures unchanged from the clean-master baseline.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

webui/app.py

@@ -29,6 +29,12 @@ from webui.worktree_scanner import load_hygiene_snapshot, snapshot_to_dict as wo
 from webui.worktree_views import render_worktrees_page
 from webui.runtime_health import load_runtime_snapshot, snapshot_to_dict as runtime_snapshot_to_dict
 from webui.runtime_views import render_runtime_page
+from webui.system_health import (
+    API_PATH as SYSTEM_HEALTH_API_PATH,
+    load_system_health,
+    process_uptime,
+    snapshot_to_dict as system_health_to_dict,
+)
 
 _READ_ONLY_METHODS = frozenset({"GET", "HEAD", "OPTIONS"})
 _AUDIT_MUTATION_PATHS = frozenset({"/audit", "/api/audit"})
@@ -62,16 +68,43 @@ async def home(_request: Request) -> HTMLResponse:
 
 
 async def health(_request: Request) -> JSONResponse:
+    """Liveness only — deliberately cheap, runs no dependency probe (#634).
+
+    Every MVP key is retained so existing pollers keep working; the additions
+    are a pointer to the structured API and the in-memory process uptime.
+    Readiness lives at that API because answering it costs real probes.
+    """
     bind_host = _request.app.state.webui_bind_host
+    started_at, uptime_seconds = process_uptime()
     return JSONResponse({
         "status": "ok",
         "service": "mcp-control-plane-webui",
         "mode": "read-only-mvp",
         "timestamp": datetime.now(timezone.utc).isoformat(),
         "deployment": deployment_snapshot(bind_host=bind_host),
+        "started_at": started_at,
+        "uptime_seconds": uptime_seconds,
+        "system_health_api": SYSTEM_HEALTH_API_PATH,
     })
 
 
+def _truthy_flag(value: str | None) -> bool:
+    return (value or "").strip().lower() in {"1", "true", "yes", "on"}
+
+
+async def api_system_health(request: Request) -> JSONResponse:
+    """Structured read-only system health (#634).
+
+    `?deep=1` opts into the expensive network probe. The response status code
+    reflects readiness so automated checks can branch on it without parsing the
+    body: 200 when ready, 503 when a required dependency failed or never ran.
+    """
+    deep = _truthy_flag(request.query_params.get("deep"))
+    snapshot = load_system_health(deep=deep)
+    payload = system_health_to_dict(snapshot)
+    return JSONResponse(payload, status_code=200 if snapshot.ready else 503)
+
+
 async def queue(_request: Request) -> HTMLResponse:
     snapshot = load_queue_snapshot()
     return HTMLResponse(render_page(title="Queue", body_html=render_queue_page(snapshot)))
@@ -263,6 +296,7 @@ def create_app(*, bind_host: str | None = None) -> Starlette:
         routes=[
             Route("/", home, methods=["GET"]),
             Route("/health", health, methods=["GET"]),
+            Route(SYSTEM_HEALTH_API_PATH, api_system_health, methods=["GET"]),
             Route("/queue", queue, methods=["GET"]),
             Route("/api/queue", api_queue, methods=["GET"]),
             Route("/projects", projects, methods=["GET"]),