Replace next-auth with Better Auth, migrate DB columns to camelCase

- Replace next-auth v5 beta with better-auth v1.6.2 (stable releases)
- Add multi-provider OAuth support with admin UI configuration
- New oauthProviders table with encrypted secrets (AES-256-GCM)
- Env var bootstrap (OAUTH_*) syncs to DB, UI-created providers fully editable
- OAuth provider REST API: GET/POST/PUT/DELETE /api/v1/oauth-providers
- Settings page "Authentication Providers" section for admin management
- Account linking uses new accounts table (multi-provider per user)
- Username plugin for credentials sign-in (replaces email@localhost pattern)
- bcrypt password compatibility (existing hashes work)
- Database-backed sessions via Kysely adapter (bun:sqlite direct)
- Configurable rate limiting via AUTH_RATE_LIMIT_* env vars
- All DB columns migrated from snake_case to camelCase
- All TypeScript types/models migrated to camelCase properties
- Removed casing: "snake_case" from Drizzle config
- Callback URL format: {baseUrl}/api/auth/oauth2/callback/{providerId}
- package-lock.json removed and gitignored (using bun.lock)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/(dashboard)/audit-log/AuditLogClient.tsx

@@ -10,7 +10,7 @@ import { PageHeader } from "@/components/ui/PageHeader";
 
 type EventRow = {
   id: number;
-  created_at: string;
+  createdAt: string;
   user: string;
   summary: string;
 };
@@ -61,7 +61,7 @@ export default function AuditLogClient({ events, pagination, initialSearch }: Pr
       width: 180,
       render: (r: EventRow) => (
         <span className="text-sm text-muted-foreground whitespace-nowrap">
-          {new Date(r.created_at).toLocaleString()}
+          {new Date(r.createdAt).toLocaleString()}
         </span>
       ),
     },
@@ -88,7 +88,7 @@ export default function AuditLogClient({ events, pagination, initialSearch }: Pr
         <div className="flex justify-between items-center">
           <Badge variant="outline">{r.user}</Badge>
           <span className="text-xs text-muted-foreground">
-            {new Date(r.created_at).toLocaleString()}
+            {new Date(r.createdAt).toLocaleString()}
           </span>
         </div>
         <p className="text-sm">{r.summary}</p>