refactor(backend): verb-first names for the provisioning modules

Rename the module files to match their functions' verb-first names:
`bottle_provision.py` -> `provision_bottle.py`,
`gateway_provision.py` -> `provision_gateway.py` (and the test file to match).
Imports, docstrings, and the git_gate/service.py pointer updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bot_bottle/git_gate/service.py

@@ -60,7 +60,7 @@ class GitGate:
         # `sh`, so the script needs the x bit. The gateway copy does not
         # necessarily preserve this mode (`docker cp` does, the Apple `container
         # cp` does not), so provisioning re-applies +x on the gateway side — see
-        # backend/gateway_provision.py.
+        # backend/provision_gateway.py.
         access_hook.chmod(0o700)
         upstreams_with_files: list[GitGateUpstream] = []
         for u in upstreams: