Add Forgejo integration

[vondutchi]

Closes PlakarKorp/hub#4.

What

Adds a new Forgejo integration with importer and exporter connectors.

• importer streams Forgejo native dumps into Plakar via forgejo dump --file - --type tar.gz --quiet by default
• supports Forgejo CLI options for binary path, work path, custom path, config path, temp dir, database syntax, dump type, and skip flags
• exporter extracts tar, tar.gz/tgz, and zip dump archives to a target directory so the snapshot content is restored to a Forgejo-loadable state
• rejects archive entries that would escape the restore target directory
• includes schemas, manifest metadata, README usage notes, and focused unit tests

Notes

Forgejo does not expose a single full-instance forgejo restore command, so this exporter restores the dump contents to disk and documents that the operator should continue with Forgejo's documented restore procedure for their database/storage backend.

Validation

From forgejo/:

go test ./...
go vet ./...
make build
make clean

[vondutchi]

Follow-up hardening pushed in 9b2e3fe:

• normalize dump_type before passing it through to forgejo dump
• handle a closed/nil importer result channel without panicking
• add focused regression tests for both cases

Verification rerun after the update:

go test ./...
go vet ./...
make build && make clean

[vondutchi]

Added one more test-only follow-up in 6837665 to cover the Forgejo command execution boundary:

• fake forgejo binary test verifies startDump streams stdout and passes the expected dump args
• Ping failure test verifies stderr is included in the returned error

Verification rerun:

go test ./...
go vet ./...
make build && make clean