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/main.py

@@ -344,15 +344,24 @@ def run_web_api():
     try:
         import uvicorn
 
-        from .web_api import app
-
         # Validate configuration
         Config.validate_config()
 
-        # Run the server
-        uvicorn.run(
-            app, host="0.0.0.0", port=8000, log_config=None  # Use our custom logging
-        )
+        workers = max(1, Config.WEB_WORKERS)
+        if workers > 1:
+            # Multi-worker needs the app as an import string; a localhost lock
+            # port keeps background collection in exactly one worker.
+            uvicorn.run(
+                "src.web_api:app",
+                host="0.0.0.0",
+                port=8000,
+                workers=workers,
+                log_config=None,
+            )
+        else:
+            from .web_api import app
+
+            uvicorn.run(app, host="0.0.0.0", port=8000, log_config=None)
 
     except ImportError:
         logger.error("FastAPI not installed. Run: pip install fastapi uvicorn")