Add OpenAPI-based API documentation and new endpoints

Integrated OpenAPI and Swagger plugins for comprehensive API documentation. Added new routes, including settings management (`/settings/*`), guild metadata endpoints (`/guilds/*`), and detailed action status handling. Updated database schema to support guild metadata persistence, revised command subcommand handling for better modularity, and enhanced API key-based authentication for all routes.

docs/web-api.md

@@ -33,7 +33,8 @@ a path prefix.
 |---|---|---|
 | `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). |
-| `ActionHandler` | `/actions` | `/actions/status` — JDA health + gateway ping; `/actions/shutdown` — graceful shutdown; `/actions/settings/current/{guildId}` — current guild settings; `/actions/settings/reload/{guildId}` — invalidate & reload the settings cache. |
+| `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. |
 
 The action/guild handlers read live state from the [`JDA`](architecture.md) instance and from the
 [persistence services](persistence.md#service-layer) (e.g. `GuildSettingService` for the settings