Normalize Caddy.Admin.Api to {:ok, _}/{:error, _} contracts (currently swallows errors)

[gsmlg]

Problem

Caddy.Admin.Api has inconsistent return contracts and silently swallows errors:

• get/1, load/1, stop/0 return %{status: 0, body: nil} on failure
• get_config/0,1, post_config, put_config, patch_config, delete_config return nil on failure
• adapt/1 returns %{} on failure
• health_check/0, get_metrics/1, server_info/0 return proper {:ok, _}/{:error, _} tuples

Callers are forced into fragile checks, e.g. map_size(json_config) > 0 in lib/caddy/server/external.ex:399.

Additional issues in the same module

• load/1 with a map does a shallow Map.merge with the current config (lib/caddy/admin/api.ex:89) — surprising semantics that silently clobber nested keys
• def api, do: nil (lib/caddy/admin/api.ex:51) is a dead placeholder holding a moduledoc
• ~400 lines of copy-pasted telemetry timing boilerplate — a single private instrument/3 helper would cut the module by two-thirds

Proposal

Converge every function on tagged tuples. This is a breaking change — batch for a 3.0 release.