SiliconDB: Edge insertion fails when source node doesn't exist

Issue

When using ingest_edges(), if the from_id references a document that doesn't exist, the entire batch fails with:

ioError("Source node not found: HR-2025-1141-A")

Expected Behavior

Either:

1. Skip invalid edges and continue with valid ones
2. Return a list of failed edges so the caller can handle them

Context

When ingesting court rulings, we create paragraph-level documents (HR-2025-1141-A/avsnitt_32) but want to create edges from case IDs (HR-2025-1141-A) to referenced laws. The edge batch fails entirely because the case-level document doesn't exist.

Workaround

Create case-level documents before inserting edges.

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