Standardize guild API endpoints and add music access checks

Unified paths in guild-related endpoints to include consistent `{guildId}` structure, improving endpoint clarity and alignment with REST routing conventions. Added centralized `MusicAccess` utility to streamline music feature permission checks, replacing scattered logic in `MusicCommand`. Updated OpenAPI documentation and added tests for new and updated behaviors.

docs/web-api.md

@@ -32,7 +32,7 @@ a path prefix.
 | Handler | Base path | Endpoints |
 |---|---|---|
 | `RootHandler` | `/` | Simple HTML "OK" landing response. |
-| `GuildHandler` | `/guilds` | `/guilds/owner` — guild ids + owner ids (JSON); `/guilds/channels/{guildId}` — text channels (JSON); `/guilds/roles/{guildId}` — roles (JSON). |
+| `GuildHandler` | `/guilds` | `/guilds/owner` — guild ids + owner ids (JSON); `/guilds/{guildId}/channels` — text channels (JSON); `/guilds/{guildId}/roles` — roles (JSON). |
 | `ActionHandler` | `/actions` | `/actions/status` — JDA health + gateway ping; `/actions/shutdown` — graceful shutdown. |
 | `SettingsHandler` | `/settings` | `/settings/catalog` — every known setting with its default value and description; `/settings/current/{guildId}` — current guild settings; `/settings/reload/{guildId}` — invalidate & reload the settings cache. |