feat: back up release notes to RELEASE_NOTES.md and Forgejo release descriptions

[notheotherben]

Introduce a Release entity that bundles a release's assets and notes as the unit of backup. Release notes are written to RELEASE_NOTES.md on the filesystem and replicated to the Forgejo release description, with source.tar.gz and RELEASE_NOTES.md excluded from Forgejo asset uploads. Filtering continues to operate per-asset via the source's internal filtering.

[codecov]

Codecov Report

❌ Patch coverage is 68.40000% with 79 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.64%. Comparing base (7c169f7) to head (b08c34f).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/engines/forgejo/release.rs	0.00%	63 Missing ⚠️
src/engines/release.rs	93.68%	6 Missing ⚠️
src/helpers/forgejo/entities.rs	63.63%	4 Missing ⚠️
src/sources/github_releases.rs	72.72%	3 Missing ⚠️
src/helpers/forgejo/client.rs	89.47%	2 Missing ⚠️
src/engines/mod.rs	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #577      +/-   ##
==========================================
+ Coverage   72.51%   72.64%   +0.13%     
==========================================
  Files          33       36       +3     
  Lines        2481     2709     +228     
==========================================
+ Hits         1799     1968     +169     
- Misses        682      741      +59     

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