feat: structural fingerprinting for incremental updates

[rajnavakoti]

Description

Hash document structure (headings, section order, key phrases) to create a fingerprint. When re-processing a document, compare fingerprints to identify which sections changed, enabling section-level incremental updates instead of full re-extraction.

Acceptance Criteria

• Fingerprint generated per document section during extraction
• Fingerprints stored alongside document metadata
• On re-feed, only changed sections re-extracted
• Cost savings reported: "3 of 12 sections changed, re-extracted only those"

Out of Scope

• Semantic similarity-based change detection
• Line-level diffing