Delete custom exercises

[rd-code-23]

Summary

• add an authenticated endpoint for deleting custom exercises
• restrict deletion to exercises owned by the current user
• add a confirmation dialog to Configure Exercise
• remove the deleted exercise from the TanStack Query cache
• remove every matching occurrence from the current routine draft and normalize positions
• return to the routine editor after successful deletion
• show inline feedback when deletion fails

Why this PR is stacked

This PR builds on the custom-exercise editing work in #93. Its base branch is therefore the #93 branch so this review shows only deletion-specific changes. After #93 is merged, this PR can be retargeted to main.

Verification

• API typecheck passes
• web typecheck passes
• focused API tests pass
• focused frontend tests pass
• API and web lint pass