feat(decomposition): add merge pass, enrichment, and Pass 0b refinements

Add obligation refinement pipeline between Pass 0a and 0b:
- Merge pass: rule-based dedup of implementation-level duplicate obligations
  within the same parent control (Jaccard similarity on action+object)
- Enrich pass: classify trigger_type (event/periodic/continuous) and detect
  is_implementation_specific from obligation text (regex-based, no LLM)
- Pass 0b: skip merged obligations, cap severity for impl-specific, override
  category to 'testing' for test obligations
- Migration 075: merged_into_id, trigger_type, is_implementation_specific
- Two new API endpoints: merge-obligations, enrich-obligations
- 30+ new tests (122 total, all passing)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend-compliance/migrations/075_obligation_refinement.sql

@@ -0,0 +1,38 @@
+-- Migration 075: Obligation Refinement Fields
+-- Supports Merge Pass (implementation-level dedup) and metadata enrichment.
+--
+-- New fields:
+--   merged_into_id    — points to survivor obligation when merged
+--   trigger_type      — event / periodic / continuous
+--   is_implementation_specific — true if obligation references concrete tool/protocol
+
+-- =============================================================================
+-- 1. Add merge tracking
+-- =============================================================================
+
+ALTER TABLE obligation_candidates
+    ADD COLUMN IF NOT EXISTS merged_into_id UUID
+        REFERENCES obligation_candidates(id);
+
+CREATE INDEX IF NOT EXISTS idx_oc_merged_into
+    ON obligation_candidates(merged_into_id)
+    WHERE merged_into_id IS NOT NULL;
+
+-- Allow 'merged' as release_state
+ALTER TABLE obligation_candidates
+    DROP CONSTRAINT IF EXISTS obligation_candidates_release_state_check;
+
+ALTER TABLE obligation_candidates
+    ADD CONSTRAINT obligation_candidates_release_state_check
+        CHECK (release_state IN ('extracted', 'validated', 'rejected', 'composed', 'merged'));
+
+-- =============================================================================
+-- 2. Add enrichment metadata
+-- =============================================================================
+
+ALTER TABLE obligation_candidates
+    ADD COLUMN IF NOT EXISTS trigger_type VARCHAR(20) DEFAULT NULL
+        CHECK (trigger_type IS NULL OR trigger_type IN ('event', 'periodic', 'continuous'));
+
+ALTER TABLE obligation_candidates
+    ADD COLUMN IF NOT EXISTS is_implementation_specific BOOLEAN DEFAULT FALSE;