fix(db,engine): cost-only event model filtering and Codebuff changed-path force parse

1. Cost-only events: Filtered out empty model names from ModelsUsed
   across SQLite, PostgreSQL, and DuckDB backends. Cost-only events
   (e.g. Codebuff) have empty model but carry CostUSD. Including them
   creates blank, unfilterable entries in model breakdowns.

2. Codebuff changed-path: Added force parse for Codebuff changed-path
   events in providerChangedPathForceParse. The composite stat-only
   freshness gate may skip same-size, same-mtime rewrites, so a concrete
   changed-path signal must always trigger the full fingerprint path.

Author: scross01

internal/db/usage.go

@@ -2108,7 +2108,11 @@ func (db *DB) GetDailyUsage(
 
 			modelNames := make([]string, 0, len(dd.models))
 			for m := range dd.models {
-				modelNames = append(modelNames, m)
+				// Filter out empty model names from cost-only events
+				// (e.g. Codebuff) to avoid blank entries in ModelsUsed.
+				if m != "" {
+					modelNames = append(modelNames, m)
+				}
 			}
 			sort.Slice(modelNames, func(i, j int) bool {
 				left := dd.models[modelNames[i]]
@@ -2275,7 +2279,11 @@ func (db *DB) GetDailyUsage(
 
 		modelNames := make([]string, 0, len(dm.models))
 		for m := range dm.models {
-			modelNames = append(modelNames, m)
+			// Filter out empty model names from cost-only events
+			// (e.g. Codebuff) to avoid blank entries in ModelsUsed.
+			if m != "" {
+				modelNames = append(modelNames, m)
+			}
 		}
 		sort.Slice(modelNames, func(i, j int) bool {
 			left := dm.models[modelNames[i]]

internal/duckdb/analytics_usage.go

@@ -3955,7 +3955,11 @@ func addUsageBucket(m map[string]duckUsageBucket, key string, b duckUsageBucket)
 func sortedUsageBucketKeys(m map[string]duckUsageBucket) []string {
 	out := make([]string, 0, len(m))
 	for key := range m {
-		out = append(out, key)
+		// Filter out empty model names from cost-only events
+		// (e.g. Codebuff) to avoid blank entries in ModelsUsed.
+		if key != "" {
+			out = append(out, key)
+		}
 	}
 	sort.Slice(out, func(i, j int) bool {
 		left := m[out[i]]

internal/postgres/usage.go

@@ -1547,7 +1547,11 @@ func (s *Store) GetDailyUsage(
 
 			modelNames := make([]string, 0, len(dd.models))
 			for m := range dd.models {
-				modelNames = append(modelNames, m)
+				// Filter out empty model names from cost-only events
+				// (e.g. Codebuff) to avoid blank entries in ModelsUsed.
+				if m != "" {
+					modelNames = append(modelNames, m)
+				}
 			}
 			sort.Slice(modelNames, func(i, j int) bool {
 				left := dd.models[modelNames[i]]
@@ -1708,7 +1712,11 @@ func (s *Store) GetDailyUsage(
 
 		modelNames := make([]string, 0, len(dm.models))
 		for m := range dm.models {
-			modelNames = append(modelNames, m)
+			// Filter out empty model names from cost-only events
+			// (e.g. Codebuff) to avoid blank entries in ModelsUsed.
+			if m != "" {
+				modelNames = append(modelNames, m)
+			}
 		}
 		sort.Slice(modelNames, func(i, j int) bool {
 			left := dm.models[modelNames[i]]

internal/sync/engine.go

@@ -1234,6 +1234,13 @@ func providerChangedPathForceParse(
 			parser.IsGeminiProjectMetadataFile(eventPath) {
 			return false
 		}
+		// Codebuff changed-path events must force a fingerprint
+		// comparison. The composite stat-only freshness gate may skip
+		// same-size, same-mtime rewrites, so a concrete changed-path
+		// signal must always trigger the full fingerprint path.
+		if agent == parser.AgentCodebuff {
+			return true
+		}
 		return true
 	}
 	return eventKind == "remove" &&