release: prepare UniversalMailCleaner 1.2.1 source

[lukisch]

Purpose

Prepare the user-authorized UniversalMailCleaner 1.2.1 source release recorded in master ticket T-20261003-933110552. This PR updates source version metadata and the cumulative changelog; it does not create a tag, GitHub release asset, executable, Store package, or installed application.

Changes

• Align pyproject.toml, the application version constant, all three README badges, llms.txt, both third-party-license version headers, and bilingual contributor freeze wording at 1.2.1.
• Move all existing Unreleased work since the actual published v1.2.0 into the dated 1.2.1 section. Explain the historical duplicate September [1.2.0] changelog heading while preserving the real v1.2.0 section dated 2026-05-02 and all older history.
• Record the merged IMAP sender/subject CR/LF/NUL validation and per-rule fail-closed behavior.
• Add metadata parity assertions for the machine-readable version surfaces.

Validation

• python -m pytest -q -p no:cacheprovider: 121 passed on Python 3.12.10.
• python -m ruff check .: passed.
• python -m compileall -q mail_imap_cleaner_v1.py imap_client.py workers.py tests: passed.
• git diff --check: passed.

The release candidate is a source-only release. After integration, the exact reviewed merge/tag commit will be packaged with git archive --format=zip --prefix=UniversalMailCleaner-1.2.1/; the ZIP will be checked for tracked-file inventory (including LICENSE, excluding .git, user profiles, databases, and keys), built twice for SHA-256 reproducibility, and accompanied by SHA256SUMS.txt. Artifact creation and publication remain pending final review. No Store or binary release is claimed.

[github-actions]

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