Add receipt export consumer for gitea-sovereign audit log

[mdheller]

Context

SocioProphet/gitea-sovereign PR #1 has merged the non-mutating PR-A scaffold for svc.substrate.source-control.

Merged baseline: SocioProphet/gitea-sovereign@5d85ab6a24502f60f15ef829235b6288a289d47e.

Requested change

Add an ingest/consumer contract for gitea-sovereign source-control receipt export batches.

Export surface needed

• export_batch_id;
• node_id;
• first_seq;
• last_seq;
• batch_hash;
• destination_ref;
• exported_at;
• acknowledgement hash and timestamp;
• duplicate export handling;
• partial export recovery.

Authority boundary

gitea-sovereign owns local audit-chain projection and emits receipt export batches. model-governance-ledger remains the canonical ledger/evidence target.

Acceptance criteria

• Consumer schema exists for source-control receipt batches.
• Ingest verifies batch idempotency.
• Ingest verifies sequence range and batch hash.
• Duplicate batches are safe and detectable.
• Partial export recovery is specified.