style: make the demo app pass ruff

Docstrings, return annotations, a named ERROR_RATE constant, targeted
S311 suppressions (randomness is jitter, not crypto), and an __init__.py
so the demo dir isn't an implicit namespace package.

Author: simonvanlierde

demo/__init__.py

@@ -0,0 +1 @@
+"""Demo telemetry source for the monitoring stack (see compose.demo.yml)."""

demo/app.py

@@ -1,26 +1,33 @@
-# Minimal FastAPI service for the demo overlay. All telemetry (traces,
-# metrics, logs with trace context) comes from OTel auto-instrumentation —
-# see compose.demo.yml. No OTel code needed here.
+"""Minimal FastAPI service for the demo overlay.
+
+All telemetry (traces, metrics, logs with trace context) comes from OTel
+auto-instrumentation — see compose.demo.yml. No OTel code needed here.
+"""
+
 import logging
 import random
 import time
 
 from fastapi import FastAPI, HTTPException
 
+ERROR_RATE = 0.1  # fixed error rate, enough to light up RED panels
+
 logging.basicConfig(level=logging.INFO)  # root logger defaults to WARNING; we want the INFO lines too
 log = logging.getLogger("demo-api")
 app = FastAPI()
 
 
 @app.get("/")
-def root():
+def root() -> dict[str, bool]:
+    """Fast, always-successful endpoint."""
     return {"ok": True}
 
 
 @app.get("/work")
-def work():
-    time.sleep(random.uniform(0.02, 0.3))
-    if random.random() < 0.1:  # NOTE: fixed 10% error rate, enough to light up RED panels
+def work() -> dict[str, bool]:
+    """Simulate variable-latency work that sometimes fails."""
+    time.sleep(random.uniform(0.02, 0.3))  # noqa: S311 — not crypto, just jitter
+    if random.random() < ERROR_RATE:  # noqa: S311
         log.error("work failed: upstream flaked")
         raise HTTPException(status_code=500, detail="upstream flaked")
     log.info("work done")