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/v1/openapi.json/route.ts

@@ -1982,7 +1982,7 @@ const spec = {
       },
       User: {
         type: "object",
-        description: "User account (password_hash is never exposed)",
+        description: "User account (passwordHash is never exposed)",
         properties: {
           id: { type: "integer" },
           email: { type: "string" },
@@ -1990,12 +1990,12 @@ const spec = {
           role: { type: "string", enum: ["admin", "user", "viewer"] },
           provider: { type: "string", example: "credentials" },
           subject: { type: "string" },
-          avatar_url: { type: ["string", "null"] },
+          avatarUrl: { type: ["string", "null"] },
           status: { type: "string", enum: ["active", "inactive"] },
-          created_at: { type: "string", format: "date-time" },
-          updated_at: { type: "string", format: "date-time" },
+          createdAt: { type: "string", format: "date-time" },
+          updatedAt: { type: "string", format: "date-time" },
         },
-        required: ["id", "email", "role", "provider", "subject", "status", "created_at", "updated_at"],
+        required: ["id", "email", "role", "provider", "subject", "status", "createdAt", "updatedAt"],
       },
       AuditLogEvent: {
         type: "object",