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)/OverviewClient.tsx

@@ -14,7 +14,7 @@ type StatCard = {
 
 type RecentEvent = {
   summary: string;
-  created_at: string;
+  createdAt: string;
 };
 
 type TrafficSummary = {
@@ -156,14 +156,14 @@ export default function OverviewClient({
                   <div className="absolute left-[28px] top-4 bottom-4 w-px bg-border" />
                   {recentEvents.map((event, index) => (
                     <div
-                      key={`${event.created_at}-${index}`}
+                      key={`${event.createdAt}-${index}`}
                       className="relative flex items-start gap-4 px-5 py-3 hover:bg-muted/30 transition-colors"
                     >
                       {/* Dot */}
                       <div className={`relative z-10 mt-1 h-3 w-3 shrink-0 rounded-full ${getEventDotColor(event.summary)}`} />
                       <span className="flex-1 text-sm leading-snug">{event.summary}</span>
                       <span className="shrink-0 text-xs text-muted-foreground tabular-nums">
-                        {formatRelativeTime(event.created_at)}
+                        {formatRelativeTime(event.createdAt)}
                       </span>
                     </div>
                   ))}