e2e: fix agent state-dir to /var/lib/restic-manager

The agent writes its encrypted secrets blob to
$DefaultSecretsPath (/var/lib/restic-manager/secrets.enc) but
the e2e fixtures created and mounted a directory at
/var/lib/restic-manager-agent — name mismatch. Result: every
`config.update` push failed with 'create tmp: no such file or
directory', the auto-init never got the repo creds, the host
landed in init_failed, and the smoke test couldn't kick off a
backup (the Run backup button is disabled while
repo_status != ready).

Align the compose volume mount and the Dockerfile mkdir on
/var/lib/restic-manager so they match the production install
script + the agent's own default.

e2e/compose.e2e.yml

@@ -60,7 +60,7 @@ services:
       # with a few files so the snapshot list isn't empty.
       - source-data:/source
       - agent-config:/etc/restic-manager
-      - agent-state:/var/lib/restic-manager-agent
+      - agent-state:/var/lib/restic-manager
     networks: [rmnet]
 
   # Playwright test runner. Profile-gated so `compose up` doesn't