chore: move settings backend to proper DI

Folds the legacy services/settingsService SQL into the Nest SettingsService
(byte-identical statements; BEGIN/COMMIT blocks become db.transaction()).
AdminService and ShareService inject it; llm-config.resolver becomes the
injectable LlmConfigResolver (SettingsService + DatabaseService), injected by
LlmParseService and PluginHostDepsFactory. No MCP surface, no bridge.

Whole-file commit, so this also carries the post-migration quirk fixes that
live in the same files: null serializes as '' (a stored "null" leaked out of
getDecryptedUserSetting as the literal string), bulk upserts skip the masked
sentinel (mirroring the single-upsert no-op) and return the written count.
Unit suite moved to tests/unit/nest with SET-SVC-001..026 preserved,
027..030 pin the fixes; settings/admin e2e converted to DI-native.

server/src/nest/admin/admin.module.ts

@@ -2,9 +2,10 @@ import { Module } from '@nestjs/common';
 import { AdminController } from './admin.controller';
 import { AdminService } from './admin.service';
 import { PluginsModule } from '../plugins/plugins.module';
+import { SettingsModule } from '../settings/settings.module';
 
 @Module({
-  imports: [PluginsModule],
+  imports: [PluginsModule, SettingsModule],
   controllers: [AdminController],
   providers: [AdminService],
 })