feat: database layer - PostgreSQL schema + memory fallback

- drizzle-orm + postgres dependencies
- Full schema: 12 tables covering all modules
- Graceful fallback: no DATABASE_URL → memory mode
- drizzle-kit config for migrations
- Memory store as generic CRUD layer
- dev.ts auto-initializes DB on startup

src/db/memory-store.ts

@@ -0,0 +1,46 @@
+/**
+ * Generic in-memory store (fallback when no PostgreSQL)
+ */
+
+type Store = Map<string, any>;
+const stores = new Map<string, Store>();
+
+function getStore(table: string): Store {
+  if (!stores.has(table)) stores.set(table, new Map());
+  return stores.get(table)!;
+}
+
+export const memoryDB = {
+  create(table: string, id: string, data: any): any {
+    const store = getStore(table);
+    const record = { ...data, id };
+    store.set(id, record);
+    return record;
+  },
+
+  getById(table: string, id: string): any | undefined {
+    return getStore(table).get(id);
+  },
+
+  list(table: string, filter?: (item: any) => boolean): any[] {
+    const items = Array.from(getStore(table).values());
+    return filter ? items.filter(filter) : items;
+  },
+
+  update(table: string, id: string, data: Partial<any>): any | null {
+    const store = getStore(table);
+    const existing = store.get(id);
+    if (!existing) return null;
+    const updated = { ...existing, ...data };
+    store.set(id, updated);
+    return updated;
+  },
+
+  delete(table: string, id: string): boolean {
+    return getStore(table).delete(id);
+  },
+
+  count(table: string, filter?: (item: any) => boolean): number {
+    return this.list(table, filter).length;
+  },
+};