chore(deps): bump python from 3.12-slim to 3.14-slim in /backend

[dependabot]

Bumps python from 3.12-slim to 3.14-slim.

[dependabot]

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[weed33834]

Closing this PR: Python 3.12 to 3.14 is a major version jump that requires thorough compatibility testing. Will upgrade incrementally (3.12 → 3.13 → 3.14) in a dedicated branch after validating.

[dependabot]

OK, I won't notify you again about this release, but will get in touch when a new version is available. If you'd rather skip all updates until the next major or minor version, let me know by commenting @dependabot ignore this major version or @dependabot ignore this minor version. You can also ignore all major, minor, or patch releases for a dependency by adding an ignore condition with the desired update_types to your config file.

If you change your mind, just re-open this PR and I'll resolve any conflicts on it.