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

src/db.rs

@@ -30,6 +30,13 @@ pub struct MemoryRecord {
     pub metadata: serde_json::Value,
     pub created_at: chrono::DateTime<chrono::Utc>,
     pub expires_at: Option<chrono::DateTime<chrono::Utc>>,
+    // Truth scoring fields (populated by background worker)
+    pub truth_value: Option<f32>,
+    pub truth_confidence: Option<f32>,
+    pub truth_category: Option<String>,
+    pub truth_evaluated_at: Option<chrono::DateTime<chrono::Utc>>,
+    pub ecan_sti: Option<f32>,
+    pub ecan_lti: Option<f32>,
 }
 
 /// Query result with similarity score
@@ -299,6 +306,13 @@ impl Database {
                     metadata: row.get("metadata"),
                     created_at: row.get("created_at"),
                     expires_at: row.get("expires_at"),
+                    // Truth fields will be populated by issue #39
+                    truth_value: None,
+                    truth_confidence: None,
+                    truth_category: None,
+                    truth_evaluated_at: None,
+                    ecan_sti: None,
+                    ecan_lti: None,
                 },
                 similarity: row.get("hybrid_score"),
                 vector_score: row.get("vector_score"),