Consolidate remaining admin workflows into /admin panel

[adrunkhuman]

Problem

The new /admin panel covers correction and exception workflows, but admin functionality is still split across the panel and the existing slash subcommands.

Today an admin still has to switch between multiple command entrypoints for common tasks such as:

• fixture creation
• fixture deletion
• results entry
• results calculation/posting
• prediction/result correction

That fragmentation makes the admin UX harder to learn and slower to use during matchday operations.

Why this matters

• A single admin hub is easier to remember under time pressure.
• Consolidation reduces context switching between slash commands and panel actions.
• It also encourages shared workflows and fewer edge-case differences between admin paths.

Goal

Move the remaining admin workflows into /admin panel so the panel becomes the primary operational surface for league management.

Scope

• Expand the panel to cover the remaining admin actions that still require standalone slash commands.
• Reuse shared service-layer logic so panel and command behavior stay consistent during the transition.
• Keep existing slash commands only as compatibility fallbacks until the panel is complete, then decide whether to deprecate them.

Candidate workflows

• fixture creation
• fixture deletion
• results entry
• results calculation
• results posting
• prediction lookup/override
• late waiver management
• result correction

Acceptance criteria

• Admins can perform the full normal matchday workflow from /admin panel.
• Panel flows are at least as clear as the existing slash-command workflows.
• Shared orchestration remains centralized rather than duplicated in UI callbacks.
• Tests cover the added panel entrypoints and critical success/error flows.