feat: add model name check and count endpoints

[shivv23]

Summary

Add two utility endpoints for model management that improve the developer and user experience.

Changes

• GET /model/check-name — Check if a model name is available before saving (scoped to project)
• GET /model/count — Get the total count of saved models (optional project_id filter)
• 6 unit tests covering available/unavailable name checks, project scoping, and count cases

Validation

• 6 new unit tests added (168 total backend tests pass)
• Single commit, rebased on latest main

[github-actions]

PR Review

Squash

Your PR has 2 commits. Please squash into a single commit.

How to fix

git fetch origin
git rebase -i origin/main   # mark all but first commit as "squash"
git push --force-with-lease

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