perf: gzip responses; multi-worker serving with single collection leader

GZipMiddleware (min 500 bytes) compresses the dashboard HTML ~4x and
station JSON up to ~100x, end-to-end through the Caddy TLS terminator —
production load testing showed the deployment is bandwidth-bound once
the response caches hit, so compression is the capacity lever.

WEB_WORKERS (default 2) runs uvicorn multi-process via the app import
string. Every worker executes the lifespan, so a localhost lock port
(COLLECTION_LEADER_PORT, default 8901) elects exactly one
background-collection leader per machine — RID/HII polling stays
once-per-cycle instead of once-per-worker; the lock releases with the
process. Locust clients now send Accept-Encoding so future runs measure
compressed transfer, as browsers do.

src/config.py

@@ -91,6 +91,11 @@ class Config:
     # TTL for the /measurements/latest response cache (hottest endpoint)
     LATEST_CACHE_TTL_SECONDS = int(os.getenv("LATEST_CACHE_TTL_SECONDS", "45"))
 
+    # Web server worker processes. Above 1, uvicorn forks workers and a
+    # localhost lock port elects a single background-collection leader.
+    WEB_WORKERS = int(os.getenv("WEB_WORKERS", "2"))
+    COLLECTION_LEADER_PORT = int(os.getenv("COLLECTION_LEADER_PORT", "8901"))
+
     # Umami analytics (self-hosted). The website id is public (it ships in the
     # dashboard <script> tag); server-side API tracking posts to /api/send.
     UMAMI_API_URL = os.getenv("UMAMI_API_URL", "https://stats.buildfor.life/api/send")