refactor: enforce layered architecture by extracting all DB queries into @sofa/db/queries/*

- Create 18 new query modules under `packages/db/src/queries/` covering every domain (availability, cache, colors, credits, cron, discovery, image-cache, imports, integrations, lists, metadata, person, settings, system-health, thumbhash, title, tracking, webhooks)
- Remove all direct `db` client imports and raw Drizzle expressions from `@sofa/core` services and `apps/server` procedures/cron; replace with named query functions
- Extract cron DB helpers (`startCronRun`, `completeCronRun`, `failCronRun`, `getLibraryTitleIds`, `getStaleLibraryTitles`, etc.) into `packages/core/src/cron.ts`
- Extract integration DB helpers into `packages/core/src/integrations.ts`
- Add `drizzle-orm` as a direct dependency of `@sofa/db` and remove it from `@sofa/core`
- Update `AGENTS.md` to document the strict layered architecture rule

packages/core/src/settings.ts

@@ -1,22 +1,19 @@
-import { db } from "@sofa/db/client";
-import { count, eq } from "@sofa/db/helpers";
-import { appSettings, user } from "@sofa/db/schema";
+import {
+  getSettingValue,
+  getUserCount as queryGetUserCount,
+  upsertSetting,
+} from "@sofa/db/queries/settings";
 
 export function getSetting(key: string): string | null {
-  const row = db.select().from(appSettings).where(eq(appSettings.key, key)).get();
-  return row?.value ?? null;
+  return getSettingValue(key);
 }
 
 export function setSetting(key: string, value: string): void {
-  db.insert(appSettings)
-    .values({ key, value })
-    .onConflictDoUpdate({ target: appSettings.key, set: { value } })
-    .run();
+  upsertSetting(key, value);
 }
 
 export function getUserCount(): number {
-  const result = db.select({ count: count() }).from(user).get();
-  return result?.count ?? 0;
+  return queryGetUserCount();
 }
 
 export function getInstanceId(): string {