Fix backup/restore test listener exposure

[DeveloperDurp]

Motivation

• The backup/restore acceptance script previously created a predictable admin account and started the server bound to :8080, which exposes a privileged test user on all interfaces during the run.
• A local/CI environment often lacks the runner sandbox, so an exposed admin account can enable remote actors to trigger deployments that execute Bash as the server user.
• The intent is to make the test safe by restricting the server to loopback, using a per-run random admin password, and adding basic startup checks so the test fails if the spawned server exits before binding.

Description

• Added loadAddr() in cmd/server/main.go and switched the server to use DURPDEPLOY_ADDR when present, preserving :8080 as the default when the env var is not set, and logging the resolved address via slog.
• Modified scripts/test-backup-restore.sh to pick a random free loopback port (127.0.0.1:<port>) and export it as SERVER_ADDR/BASE for the test instead of using :8080/localhost:8080.
• Replaced the fixed test admin password with a generated per-run secret (openssl rand -base64 32) and pass DURPDEPLOY_ADDR into the test server invocation so it binds only to the loopback address chosen by the test.
• Hardened the script's server-wait loop to verify the spawned server process is still running while awaiting the bind and to fail early with server logs if the server exits before binding; applied the same check for the restart after restore.

Testing

• Ran shell syntax check bash -n scripts/test-backup-restore.sh and it succeeded. (pass)
• Verified formatting with gofmt -l cmd/server/main.go and git diff --check, both returned clean results. (pass)
• Attempted go test -v -count=1 ./... but it could not complete because generated *_templ.go files and static/swagger-ui/ are absent and templ installation is blocked by network/proxy restrictions in this environment; templ generate could not be run here. (blocked)
• make swagger-ui-copy produced the Swagger UI assets but templ-generate failed locally due to the missing templ binary; subsequent go test remains un-run for the reasons above. (partial/blocked)

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Codex Task