Preserve real user IDs for admins instead of collapsing them into a shared "admin" account

[rioredwards]

Summary

Admin users are currently rewritten to the literal user ID admin during sign-in and JWT creation. That collapses all configured admins into one logical account.

Why this matters

• Multiple admins would share the same profile row and uploaded-photo ownership
• Auditability is lost because actions can no longer be tied to the real provider account
• Any future per-admin preferences or profile edits will overwrite each other

Evidence

• app/auth.ts:34-37
• app/auth.ts:98-103

Suggested fix

Keep each user's canonical provider-based ID, then store admin status as authorization only. For example, preserve github-123 as the user ID and compute isAdmin separately in session or server-side authorization checks.

Acceptance criteria

• Admin users keep unique user IDs
• Admin privileges still work for delete and moderation paths
• Existing profile and photo ownership logic continues to work per user
• Add regression coverage for at least two admin accounts