Batch subject creation API with pre-minted IDs for interlinked imports

[JeroenDeDauw]

Problem

The REST API creates Subjects one at a time. Importing a set of Subjects that reference each other via relations
therefore requires every target's ID to exist before the referencing Subject can be written — forcing a Wikibase-style
two-pass import (create all Subjects, then add the cross-references in a second pass).

Wanted

A batch-create operation that mints IDs at import time, so interlinked Subjects import in one operation. Either:

• client-declared placeholder IDs resolved server-side within the batch, or
• the server returning minted IDs for the whole batch before statements are attached.

Validation applies per relation as in #1082.

Interactions

• Subject Sources (source, localId) identity: Implement the Subject Sources foundation (ADR 23) #993.
• Stub target nodes (Keep referenced Subjects as stubs instead of deleting them #1080) already cover in-batch forward references.

Driver

Bulk import as a first-class capability for cultural-heritage datasets (ECHOLOT import/export work package).

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_{AI-authored — Claude Code, Opus 4.8 (max); detailed spec from @JeroenDeDauw via an orchestrating agent; not yet human-reviewed; code/doc citations and cross-referenced issues/PRs verified against origin/master and the GitHub API.}

[JeroenDeDauw]

Closing: this landed before it was filed. #1100 / #1101 (merged 2026-07-20) added batch POST /neowiki/v0/subject-ids minting plus client-supplied IDs on the create endpoints, which covers the interlinked-import need described here: mint IDs up front, then create subjects that forward-reference them. This issue was filed from a research snapshot predating that merge. The one part not covered — a single-call batch-create endpoint — has no concrete demand; it can get its own issue if that changes.

_{AI-authored — Claude Code, Fable 5 (max); self-initiated correction during @JeroenDeDauw-directed work, no steering; not human-reviewed; duplicate verified against the merged #1101 endpoint description and closed #1100.}