fix(settings): map removed memcached.MemcachedCache to PyMemcacheCache

[alexzhangs]

Problem

The Django 5.2 upgrade (#49) switched the memcached driver to pymemcache and supports SSM_CACHES_BACKEND=memcached.PyMemcacheCache. But deployments upgraded in place from Django 3.x still have SSM_CACHES_BACKEND=memcached.MemcachedCache persisted in their .ssm-env — and that backend was removed in Django 4.1. On Django 5 they raise InvalidCacheBackendError and the manager won't boot.

(Confirmed on the live aiview.com hub: its .ssm-env carries SSM_CACHES_BACKEND=memcached.MemcachedCache.)

Fix

Transparently remap the dropped alias memcached.MemcachedCache → memcached.PyMemcacheCache (with a DeprecationWarning), so existing deployments survive the Django 5 image without manual env surgery. The memcached.* LOCATION wiring already added in #49 then applies normally.

Note

This also matters for the cross-process django-cache-lock the EIP-rotation tasks use: it needs the shared memcached cache, so silently falling back to anything else (or failing to boot) would be worse than remapping.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.51%. Comparing base (03315c2) to head (e90b8c0).

Files with missing lines	Patch %	Lines
...hadowsocks_manager/shadowsocks_manager/settings.py	0.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #59      +/-   ##
===========================================
- Coverage    93.64%   93.51%   -0.14%     
===========================================
  Files           53       53              
  Lines         2850     2854       +4     
  Branches       238      239       +1     
===========================================
  Hits          2669     2669              
- Misses         112      115       +3     
- Partials        69       70       +1     

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