feat: add /health endpoint for microservice monitoring

Checks database, background workers (ingestion, collector, attacker,
sniffer), and Docker daemon. Reports healthy/degraded/unhealthy status
with per-component details. Returns 503 when required services fail,
200 for healthy or degraded (only optional services down).

decnet/web/router/health/api_get_health.py

@@ -0,0 +1,80 @@
+from typing import Any
+
+from fastapi import APIRouter, Depends
+from fastapi.responses import JSONResponse
+
+from decnet.web.dependencies import require_viewer, repo
+from decnet.web.db.models import HealthResponse, ComponentHealth
+
+router = APIRouter()
+
+_OPTIONAL_SERVICES = {"sniffer_worker"}
+
+
+@router.get(
+    "/health",
+    response_model=HealthResponse,
+    tags=["Observability"],
+    responses={
+        401: {"description": "Could not validate credentials"},
+        503: {"model": HealthResponse, "description": "System unhealthy"},
+    },
+)
+async def get_health(user: dict = Depends(require_viewer)) -> Any:
+    components: dict[str, ComponentHealth] = {}
+
+    # 1. Database
+    try:
+        await repo.get_total_logs()
+        components["database"] = ComponentHealth(status="ok")
+    except Exception as exc:
+        components["database"] = ComponentHealth(status="failing", detail=str(exc))
+
+    # 2. Background workers
+    from decnet.web.api import get_background_tasks
+    for name, task in get_background_tasks().items():
+        if task is None:
+            components[name] = ComponentHealth(status="failing", detail="not started")
+        elif task.done():
+            if task.cancelled():
+                detail = "cancelled"
+            else:
+                exc = task.exception()
+                detail = f"exited: {exc}" if exc else "exited unexpectedly"
+            components[name] = ComponentHealth(status="failing", detail=detail)
+        else:
+            components[name] = ComponentHealth(status="ok")
+
+    # 3. Docker daemon
+    try:
+        import docker
+
+        client = docker.from_env()
+        client.ping()
+        client.close()
+        components["docker"] = ComponentHealth(status="ok")
+    except Exception as exc:
+        components["docker"] = ComponentHealth(status="failing", detail=str(exc))
+
+    # Compute overall status
+    required_failing = any(
+        c.status == "failing"
+        for name, c in components.items()
+        if name not in _OPTIONAL_SERVICES
+    )
+    optional_failing = any(
+        c.status == "failing"
+        for name, c in components.items()
+        if name in _OPTIONAL_SERVICES
+    )
+
+    if required_failing:
+        overall = "unhealthy"
+    elif optional_failing:
+        overall = "degraded"
+    else:
+        overall = "healthy"
+
+    result = HealthResponse(status=overall, components=components)
+    status_code = 503 if overall == "unhealthy" else 200
+    return JSONResponse(content=result.model_dump(), status_code=status_code)