feat(stage6): source independence & citation graph — detect syndication, shared origins, text similarity
- SourceIndependenceModel: per-source independence_score (0.0-1.0), syndication_group_id, primary_source_id, content_hash, shared_urls
- CitationGraphEdgeModel: directed edges (SYNDICATED, QUOTES, LINKS_TO, REPOST, SIMILAR_CONTENT) with confidence + evidence
- Content-Hash (SHA-256): instant syndication detection for identical content
- difflib Vorfilterung: >60% → LLM, >80% → high confidence, 100% → immediate syndication
- LLM-Pairwise-Analysis: two-text-comparison for suspicious pairs only (bounded concurrency)
- independence_score: 1.0 base, -0.4 for syndicated, -0.1 per high-similarity pair
- Pydantic schemas: SourceIndependenceScore, CitationGraphEdge, SyntacticSimilarityResult, LlmSyndicationAnalysis, SourceIndependenceAnalysisResult
- LLM response parser: handles JSON, markdown code blocks, partial/invalid JSON
- 43 tests: content hash, similarity thresholds, LLM parsing, analyzer integration, edge cases, prompt templates
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