Add load test harness for pool and fleet sizing

Adds the scripts used to size the connection pool and validate fleet polling under load: burst arrival and mixed workload drivers, pool and resource samplers, a UI probe, host seed/drop SQL, and a gevent Dockerfile. Hosts are addressed by compose service name (api:5001) rather than a local path, so the harness runs against any stack rather than one machine.

loadtest_token.py mints a scoped collector token named LOADTEST and prints the secret once, then revokes only that token by name so a real collector token is never touched. envkeyfallback is false, so the harness needs a genuine managed token rather than an env key.

scripts/loadtest/sample_dbconns.py

@@ -0,0 +1,68 @@
+"""Sample live MySQL connection counts at high frequency.
+
+WHY: pool pressure from a long-poll fleet is BURSTY. The route releases its
+connection before sleeping and re-acquires on wake, so the interesting number is
+the peak concurrent checkout during a wake burst, not the steady-state count. A
+slow sampler misses it entirely.
+
+Runs on the HOST (needs the docker CLI).
+
+Usage:
+    python sample_dbconns.py --seconds 120 --interval 0.25
+"""
+import argparse
+import json
+import statistics
+import subprocess
+import time
+
+QUERY = ("SELECT COUNT(*), SUM(command<>'Sleep') "
+         "FROM information_schema.processlist WHERE user='shopdb';")
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--container', default='shopdb-flask-wilm-db-1')
+    parser.add_argument('--seconds', type=int, default=120)
+    parser.add_argument('--interval', type=float, default=0.25)
+    arguments = parser.parse_args()
+
+    total_samples = []
+    active_samples = []
+    deadline = time.monotonic() + arguments.seconds
+    command = ('mysql -u root -p$MYSQL_ROOT_PASSWORD -N -B -e "%s"' % QUERY)
+
+    while time.monotonic() < deadline:
+        try:
+            output = subprocess.check_output(
+                ['docker', 'exec', arguments.container, 'sh', '-c', command],
+                stderr=subprocess.DEVNULL, text=True).strip()
+        except subprocess.CalledProcessError:
+            break
+        if not output:
+            break
+        parts = output.split()
+        total_samples.append(int(parts[0]))
+        active_samples.append(int(parts[1]) if parts[1] != 'NULL' else 0)
+        time.sleep(arguments.interval)
+
+    def summarize(values):
+        if not values:
+            return {'samples': 0}
+        ordered = sorted(values)
+        index = min(int(len(ordered) * 0.95), len(ordered) - 1)
+        return {
+            'samples': len(values),
+            'median': statistics.median(values),
+            'p95': ordered[index],
+            'max': max(values),
+        }
+
+    print(json.dumps({
+        'connections': summarize(total_samples),
+        'active_queries': summarize(active_samples),
+    }, indent=2))
+
+
+if __name__ == '__main__':
+    main()