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/pool_status.py

@@ -0,0 +1,42 @@
+"""Report SQLAlchemy pool saturation from inside the API container.
+
+WHY: MySQL's max_connections is NOT the ceiling that binds first. The effective
+limit is per-process: pool_size + max_overflow, multiplied by worker count.
+With the defaults (5 + 10) and 4 workers that is 60, not 151. A peak of 52
+therefore means 87 percent of the real ceiling, not 34 percent.
+
+Prints the computed ceiling so a measured high-water mark can be compared
+against the right number.
+
+Usage (inside the api container, PYTHONPATH=/app):
+    python pool_status.py
+"""
+import json
+import os
+
+from shopdb import create_app
+from shopdb.api import db
+
+
+def main():
+    app = create_app(os.environ.get('FLASK_ENV', 'production'))
+    with app.app_context():
+        engine = db.engine
+        pool = engine.pool
+        size = pool.size()
+        overflow = getattr(pool, '_max_overflow', 0)
+        workers = int(os.environ.get('GUNICORN_WORKERS', '4'))
+        print(json.dumps({
+            'pool_class': type(pool).__name__,
+            'pool_size': size,
+            'max_overflow': overflow,
+            'per_worker_ceiling': size + overflow,
+            'workers': workers,
+            'effective_ceiling': (size + overflow) * workers,
+            'checkedout': pool.checkedout(),
+            'status': pool.status(),
+        }, indent=2))
+
+
+if __name__ == '__main__':
+    main()