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/force_pool.py

@@ -0,0 +1,22 @@
+"""Force SQLAlchemy pool sizing for a pool-comparison run.
+
+WHY this exists: shopdb/config.py does not read pool size from the environment,
+so a test container cannot be pointed at a different pool without a code change.
+This patches Config.SQLALCHEMY_ENGINE_OPTIONS at import time, before create_app
+builds the engine, purely so a bigger pool can be MEASURED before deciding
+whether to make it permanent.
+
+Not for production use. Mounted into a throwaway container as sitecustomize.py,
+which Python imports automatically at startup.
+"""
+import os
+
+from shopdb.config import Config
+
+_size = int(os.environ.get('FORCE_POOL_SIZE', '5'))
+_overflow = int(os.environ.get('FORCE_MAX_OVERFLOW', '10'))
+
+_options = dict(Config.SQLALCHEMY_ENGINE_OPTIONS)
+_options['pool_size'] = _size
+_options['max_overflow'] = _overflow
+Config.SQLALCHEMY_ENGINE_OPTIONS = _options