chore(slack): remove dead invite path, archive checkSlackJoins poller

Wave-based onboarding makes the auto-invite + polling path obsolete.

- Removes SlackService.inviteUserToSlack — admins now send invites
  through Slack's UI and flip the flag in our admin endpoint.
- Removes the slack_invite_failed admin alert + its detector. The
  alert no longer has a meaningful trigger (we don't attempt invites).
- Archives server/utils/checkSlackJoins.js (and its test) under
  _archive/ in case the polling pattern is needed again post-pilot.
- Deletes the Nitro plugin that scheduled checkSlackJoins on boot
  + hourly. Nothing in nitro.config / nuxt.config / package.json
  registered it elsewhere.
- Drops the slack_invite_failed branch from adminAlerts.test; the
  enum slug stays in adminAlertDismissal so historical dismissal
  rows continue to validate.

notifyNewMember (vetting-channel notification) and findUserByEmail
(used by the auto-flag helper) are retained.

server/utils/slack.ts

@@ -9,98 +9,6 @@ export class SlackService {
     this.vettingChannelId = vettingChannelId;
   }
 
-  /**
-   * Invite user to workspace and channel (using proper admin and conversation scopes)
-   */
-  async inviteUserToSlack(
-    email: string,
-    realName: string,
-  ): Promise<{
-    success: boolean;
-    userId?: string;
-    status?: string;
-    error?: string;
-  }> {
-    try {
-      // First, check if user already exists in workspace
-      const existingUser = await this.findUserByEmail(email);
-
-      if (existingUser) {
-        // User exists, invite them to the vetting channel
-        try {
-          await this.client.conversations.invite({
-            channel: this.vettingChannelId,
-            users: existingUser,
-          });
-
-          console.log(
-            `Successfully invited existing user ${email} to vetting channel`,
-          );
-          return {
-            success: true,
-            userId: existingUser,
-            status: "existing_user_added_to_channel",
-          };
-        } catch (error: any) {
-          if (error.data?.error === "already_in_channel") {
-            return {
-              success: true,
-              userId: existingUser,
-              status: "user_already_in_channel",
-            };
-          }
-          throw error;
-        }
-      }
-
-      // User doesn't exist, try to invite to workspace using admin API
-      try {
-        const inviteResponse = await this.client.admin.users.invite({
-          email: email,
-          real_name: realName,
-          channel_ids: [this.vettingChannelId],
-          is_restricted: true, // Single-channel guest
-          is_ultra_restricted: false,
-        });
-
-        if (inviteResponse.ok && inviteResponse.user) {
-          console.log(
-            `Successfully invited ${email} to workspace as single-channel guest`,
-          );
-          return {
-            success: true,
-            userId: inviteResponse.user.id,
-            status: "new_user_invited_to_workspace",
-          };
-        } else {
-          throw new Error(`Admin invite failed: ${inviteResponse.error}`);
-        }
-      } catch (adminError: any) {
-        console.log(
-          `Admin API not available or failed: ${
-            adminError.data?.error || adminError.message
-          }`,
-        );
-
-        // Fall back to manual process
-        return {
-          success: true,
-          status: "manual_invitation_required",
-          error: `Admin API unavailable: ${
-            adminError.data?.error || adminError.message
-          }`,
-        };
-      }
-    } catch (error: any) {
-      console.error(`Failed to process invitation for ${email}:`, error);
-
-      return {
-        success: false,
-        error: error.data?.error || error.message || "Unknown error occurred",
-      };
-    }
-  }
-
   /**
    * Find user in workspace by email
    */