ax(mining): replace prose struct-field comments with usage examples

DatabaseConfig field comments restated the type signature in prose
(AX §2 violation). Replaced with concrete assignment examples showing
real values and edge-case behaviour (0 → defaults to 30).

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/config_manager.go

@@ -22,9 +22,9 @@ type MinerAutostartConfig struct {
 
 // DatabaseConfig{Enabled: true, RetentionDays: 30}
 type DatabaseConfig struct {
-	// Enabled determines if database persistence is active (default: true)
+	// cfg.Enabled = false // disable hashrate persistence entirely
 	Enabled bool `json:"enabled"`
-	// RetentionDays is how long to keep historical data (default: 30)
+	// cfg.RetentionDays = 90 // purge rows older than 90 days (0 → defaults to 30)
 	RetentionDays int `json:"retentionDays,omitempty"`
 }