fix: switch dispatcher classifier to gpt-5.4-nano

gpt-5.4-nano correctly discriminates complexity (1 vs 10)
while deepseek-v4-flash rated everything as 1/10.

internal/db/db.go

@@ -205,7 +205,7 @@ func (db *DB) RunMigrations() error {
 		{"heavy-logic", "heuristic", `["grok-4.3","kimi-k2.6","deepseek-v4-pro"]`, "", nil, intPtr(9), strPtr("Complex Coding, Logic, Agents.")},
 		{"standard-pro", "heuristic", `["gpt-5.4-mini","gemini-3-flash-preview"]`, "", nil, intPtr(5), strPtr("General Assistant, Long Docs.")},
 		{"fast-flow", "heuristic", `["deepseek-v4-flash","gpt-5.4-nano"]`, "", nil, intPtr(2), strPtr("Classification, JSON, Basic Q&A.")},
-		{"dispatcher", "classifier", `["fast-flow","standard-pro","heavy-logic"]`, "deepseek-v4-flash", intPtr(10), nil, strPtr("Auto-dispatches to tier groups by complexity.")},
+		{"dispatcher", "classifier", `["fast-flow","standard-pro","heavy-logic"]`, "gpt-5.4-nano", intPtr(10), nil, strPtr("Auto-dispatches to tier groups by complexity.")},
 	}
 	for _, g := range defaultGroups {
 		db.Exec(`INSERT OR IGNORE INTO model_groups (id, strategy, targets, selector_model, complexity_threshold, logic_level, primary_use) VALUES (?, ?, ?, ?, ?, ?, ?)`,