Versioned extraction runs: preserve prior entities + track which backend/model produced each one

[JordanCoin]

Problem

Re-extracting a document overwrites prior entities. There is no run log. We lose:

• Audit trail — after a Claude-Opus re-extraction, we can't see what Claude-Haiku said last week
• Reproducibility — no way to tell which extraction tier or which model produced a given entity
• Regression detection — can't diff extractions across model upgrades or prompt changes
• Dispute resolution — if a reporter challenges an entity, we can't show the provenance

Currently: Document.entities_extracted: bool — just a gate, not a log.

Not a duplicate of #60

#60 redesigns the extraction architecture (parallel extractors → LLM validates). This issue is about what we persist regardless of which architecture wins. Even after #60 lands, we still overwrite.

Proposed shape

New table — something like extraction_runs:

column	purpose
id	UUID
document_id	FK
backend	llm / gliner / spacy / regex / ensemble
model	e.g. claude-opus-4-6, gliner-large-v2.1, en_core_web_trf
prompt_hash	for LLM runs — hash of the system prompt used
started_at / completed_at	timing
entity_count	quick stat
notes	free text (e.g. "validation kept 5, removed 2")

Then: Entity.extraction_run_id (FK, nullable for legacy rows).

Migration / back-compat

• Add extraction_runs table (Alembic migration)
• Add nullable extraction_run_id on entities
• Existing entities have extraction_run_id = NULL (legacy)
• New extractions write a run row first, then entities reference it
• `--force` re-extract: don't delete prior entities; just insert a new run with new entities. Dedup at query time using canonical_id.

UI / CLI changes

• `openfoia analyze runs ` — list runs for a document
• `openfoia analyze extract --prefer-run ` — pick which run the graph/crossref uses
• Graph view: show badge for backend/model per entity

Out of scope for this issue

• Versioning the documents themselves (separate concern — docs change too, e.g. re-OCR)
• Versioning entity links — start with just entities