fix(GAT-9018): free-text linkage fallback + IndexElastic delta-row fixes

[calmacx]

Stacked on feat/GAT-9018-2. Reinstates the fixes/features that were split out of PR2 because they're independent of the version-handler/DatasetService rearrangement — genuinely pre-existing bugs or new functionality, not caused by the refactor:

• Free-text linkage fallback: dataset/publication linkages that can't be resolved to a real row are now stored with their raw external url/pid/title/doi instead of being silently dropped. New raw_url/raw_pid/raw_title columns on dataset_version_has_dataset_version, raw_doi on publication_has_dataset_version, both FKs made nullable. Gwdm2xHandler::processDatasetLinkages()/processPublicationLinkages() write the unresolved fallback; afterRead() and DatasetService::getLinkages() read it back.
• IndexElastic delta-row fix: delta dataset_version rows store an empty metadata column by design, but ES indexing and the tool/publication extraction jobs read it directly, crashing or silently no-oping on delta rows. They now reconstruct the full envelope via DatasetService::getReconstructedMetadataEnvelope(). Adds GwdmMetadataHandler::supportsElasticIndexing() so datasets on a non-indexable GWDM version get deindexed rather than partially indexed, and moves Typesense field extraction into the handler hierarchy via toSearchableFields().
• Spatial-coverage pivot pruning fix: mapCoverage() and Dataset::getSpatialCoverageViaVersions() now scope to the latest version and prune stale pivot rows, so editing coverage down actually removes entries instead of accumulating across versions forever.

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