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.

scripts/locustfile.py

@@ -20,6 +20,9 @@ import random
 
 from locust import FastHttpUser, between, task
 
+# Explicit so measurements reflect compressed transfer (browsers always send this)
+GZIP = {"Accept-Encoding": "gzip, deflate"}
+
 
 class DashboardVisitor(FastHttpUser):
     """A browser session: initial page load, then periodic refresh polling."""
@@ -29,23 +32,23 @@ class DashboardVisitor(FastHttpUser):
 
     def on_start(self):
         # What one real page load requests
-        self.client.get("/")
-        self.client.get("/stations")
-        self.client.get("/measurements/latest?limit=500")
-        self.client.get("/api/hii/waterlevel/latest")
-        self.client.get("/api/hii/rainfall/latest")
+        self.client.get("/", headers=GZIP)
+        self.client.get("/stations", headers=GZIP)
+        self.client.get("/measurements/latest?limit=500", headers=GZIP)
+        self.client.get("/api/hii/waterlevel/latest", headers=GZIP)
+        self.client.get("/api/hii/rainfall/latest", headers=GZIP)
 
     @task(4)
     def poll_latest(self):
-        self.client.get("/measurements/latest?limit=500")
+        self.client.get("/measurements/latest?limit=500", headers=GZIP)
 
     @task(2)
     def poll_forecast(self):
-        self.client.get("/forecast")
+        self.client.get("/forecast", headers=GZIP)
 
     @task(2)
     def poll_rain(self):
-        self.client.get("/api/hii/rainfall/latest")
+        self.client.get("/api/hii/rainfall/latest", headers=GZIP)
 
     @task(1)
     def view_history(self):
@@ -53,12 +56,13 @@ class DashboardVisitor(FastHttpUser):
         hours = random.choice([24, 168, 720])
         self.client.get(
             f"/measurements/history/{station}?hours={hours}",
+            headers=GZIP,
             name="/measurements/history/[station]",
         )
 
     @task(1)
     def stats(self):
-        self.client.get("/api/stats")
+        self.client.get("/api/stats", headers=GZIP)
 
 
 class ApiConsumer(FastHttpUser):
@@ -69,25 +73,26 @@ class ApiConsumer(FastHttpUser):
 
     @task(3)
     def latest(self):
-        self.client.get("/measurements/latest?limit=100")
+        self.client.get("/measurements/latest?limit=100", headers=GZIP)
 
     @task(3)
     def hii_feeds(self):
         self.client.get(random.choice(
             ["/api/hii/waterlevel/latest", "/api/hii/rainfall/latest"]
-        ), name="/api/hii/[feed]/latest")
+        ), headers=GZIP, name="/api/hii/[feed]/latest")
 
     @task(2)
     def forecast(self):
-        self.client.get("/forecast")
+        self.client.get("/forecast", headers=GZIP)
 
     @task(2)
     def heavy_history(self):
         self.client.get(
             "/measurements/history/P.1?hours=8760",
+            headers=GZIP,
             name="/measurements/history/P.1 [heavy]",
         )
 
     @task(1)
     def health(self):
-        self.client.get("/health")
+        self.client.get("/health", headers=GZIP)