Add Forgejo integration scaffold

[Alex-Parejo]

This is a scope-alignment draft for PlakarKorp/hub#4, not a final bounty claim yet.

Summary

• Adds a new forgejo integration package with importer/exporter plugins.
• Importer runs forgejo dump --type zip into a temporary directory and streams the resulting archive into Plakar as /forgejo-dump.zip.
• Exporter restores the saved dump archive to output_dir so an administrator can run the Forgejo restore procedure for their deployment.
• Adds schema files, manifest, README, Makefile, and focused unit tests.

Verification

C:\Users\Alex\Documents\Codex\2026-06-02\goal-generame-5-dolares-y-haz\work\tools\go\bin\go.exe fmt ./...
C:\Users\Alex\Documents\Codex\2026-06-02\goal-generame-5-dolares-y-haz\work\tools\go\bin\go.exe test ./...
C:\Users\Alex\Documents\Codex\2026-06-02\goal-generame-5-dolares-y-haz\work\tools\go\bin\go.exe build ./...

Scope questions

I opened scope questions on hub#4 before claiming. This draft assumes a tier3 first version means local forgejo dump backup plus archive export/restore staging, without remote SSH/container orchestration or direct destructive restore commands.