Remove beaker dependency, switch to Pyramid SignedCookieSessionFactory

Replace server-side session backends (redis, file, database, memcached)
with client-side signed cookie sessions. Users will need to
re-authenticate after deploy.

- Rewrite rc_beaker.py to use pyramid.session.SignedCookieSessionFactory
- Gut user_sessions.py to cookie-only stub
- Drop beaker.container from memory_lru_dict.py (pure repoze.lru)
- Migrate ini configs from beaker.session.* to session.* namespace
- Remove beaker==1.13.0 from requirements.txt
- Remove beaker.backends entry points from pyproject.toml
- Preserve backward-compat fallbacks for encryption key resolution

rhodecode/model/db.py

@@ -117,7 +117,7 @@ log = logging.getLogger(__name__)
 # =============================================================================
 
 # this is propagated from .ini file rhodecode.encrypted_values.secret or
-# beaker.session.secret if first is not set.
+# session.secret if first is not set.
 # and initialized at environment.py
 ENCRYPTION_KEY: bytes = b""