feat(db): add truth scoring columns migration (#31)

Add V5__truth_scoring.sql migration:
- truth_value (REAL): 0.0-1.0 truth score from PLN reasoning
- truth_confidence (REAL): 0.0-1.0 confidence in the score
- truth_category (TEXT): verified/plausible/unverified/contradicted
- truth_evaluated_at (TIMESTAMPTZ): when last scored
- ecan_sti (REAL): Short-Term Importance (recency-weighted)
- ecan_lti (REAL): Long-Term Importance (reliability)

Partial indexes for efficient unscored memory queries.
Update MemoryRecord struct with optional truth fields.

Part of #29

migrations/V5__truth_scoring.sql

@@ -0,0 +1,34 @@
+-- V5: Add truth scoring columns for the Truth Engine integration
+-- Part of: https://gitea.ingwaz.work/Ingwaz/openbrain-mcp/issues/31
+--
+-- All columns are nullable — existing memories start unscored and
+-- get populated by the background truth scoring worker.
+--
+-- Rollback (manual):
+--   ALTER TABLE memories
+--     DROP COLUMN IF EXISTS truth_value,
+--     DROP COLUMN IF EXISTS truth_confidence,
+--     DROP COLUMN IF EXISTS truth_category,
+--     DROP COLUMN IF EXISTS truth_evaluated_at,
+--     DROP COLUMN IF EXISTS ecan_sti,
+--     DROP COLUMN IF EXISTS ecan_lti;
+--   DROP INDEX IF EXISTS idx_memories_truth_category;
+--   DROP INDEX IF EXISTS idx_memories_truth_unevaluated;
+
+ALTER TABLE memories
+  ADD COLUMN truth_value        REAL,
+  ADD COLUMN truth_confidence    REAL,
+  ADD COLUMN truth_category      TEXT,
+  ADD COLUMN truth_evaluated_at  TIMESTAMPTZ,
+  ADD COLUMN ecan_sti            REAL,
+  ADD COLUMN ecan_lti            REAL;
+
+-- Partial index: quickly find memories by truth category
+CREATE INDEX idx_memories_truth_category
+  ON memories (truth_category)
+  WHERE truth_category IS NOT NULL;
+
+-- Partial index: efficiently find unscored memories for the background worker
+CREATE INDEX idx_memories_truth_unevaluated
+  ON memories (created_at)
+  WHERE truth_evaluated_at IS NULL;