CI: verify example generation is idempotent

[Copilot]

Adds a check-examples job that regenerates all three example projects (Django, Flask, FastAPI) and fails if any tracked files changed, excluding lockfiles.

• Runs each example.sh in a subshell, then checks with git diff --exit-code -- . ':!**/poetry.lock'
• Runs in parallel with the existing test job (no DB dependency needed)

Original prompt

add an action that test that generating example yelds no changes (except changes in lockfiles, which is allowable). do it with git diff --exit-code

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