Warnings point at library files instead of the caller (stacklevel)

[allen0099]

Problem

Several warnings point at a library file instead of the user's code, so the "where did this come from" line is useless:

• the Memcached RuntimeWarnings for clear_pattern / get_all_keys point at manager.py or routes.py when they are raised through CacheManager or the monitoring routes;
• the JWT short-secret UserWarning points at session/manager.py (SessionManager.__init__ builds the serializer).

Proposal

Check the stacklevel of every warnings.warn in the package against its real call paths. Where the depth varies, compute it (skip frames inside fastapi_cachex) instead of hard-coding it. Add tests that assert the warning's filename is the test file.