pool: stop and remove orphan controller services on supervisor startup

[aliel]

Forgotten VMs left behind stale controller.json files and sometimes
running aleph-vm-controller@.service units with active qemu
processes that kept consuming host RAM. The admission check iterates
pool.executions and did not see these orphans, so the host's real
free memory was lower than the check computed.

On load_persistent_executions, scan EXECUTION_ROOT for controller
configs whose vm_hash is not in the pool, stop_and_disable the
matching systemd service, and delete the config file.

[codecov]

Codecov Report

❌ Patch coverage is 3.57143% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.35%. Comparing base (5e501fb) to head (a4cbc7e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/aleph/vm/pool.py	3.57%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #933      +/-   ##
==========================================
- Coverage   68.48%   68.35%   -0.13%     
==========================================
  Files         104      104              
  Lines       11955    11983      +28     
  Branches     1020     1023       +3     
==========================================
+ Hits         8187     8191       +4     
- Misses       3503     3527      +24     
  Partials      265      265              

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