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/api/user/update-avatar/route.ts

@@ -47,7 +47,7 @@ export async function POST(request: NextRequest) {
 
     // Update user avatar
     const updatedUser = await updateUserProfile(userId, {
-      avatar_url: avatarUrl
+      avatarUrl: avatarUrl
     });
 
     if (!updatedUser) {
@@ -69,7 +69,7 @@ export async function POST(request: NextRequest) {
 
     return NextResponse.json({
       success: true,
-      avatarUrl: updatedUser.avatar_url
+      avatarUrl: updatedUser.avatarUrl
     });
   } catch (error) {
     console.error("Avatar update error:", error);