Deprecate legacy public APIs in backend/src/legacy

[Li-AmG]

Summary

Deprecate the public legacy APIs requested in issue cuentaprueba244w-dotcom#4 and add per-file deprecation tallies.

Changes

• Added #[deprecated(note = "Use v2::stream instead")] to all public functions in backend/src/legacy/mod.rs, migrations.rs, deprecations.rs, and v1_compat.rs.
• Added deprecated public function count comments at the top of each touched file.
• Kept the change scoped to the legacy backend compatibility surface.

Testing

• Ran python build.py -m backend with the local Windows Python environment.
• The build completed far enough to generate diagnostics, but the environment is missing Rust/toolchain binaries so the backend build reports command-not-found failures.
• Diagnostic artifacts were generated in diagnostic/build-00000000.json and diagnostic/build-00000000.logd.

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR
• Documentation has been updated, if applicable
• Configuration or schema changes are documented, if applicable
• No generated build artifacts are committed, except the required diagnostic build log
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered

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• I would like to request that my diagnostic build log is removed before merging