🔧 feat: add OpenTelemetry instrumentation to persistence layer

- Added persistence telemetry configuration option (telemetry.persistence.enabled)
- Created PersistenceTelemetryConfig struct for fine-grained control
- Added GetPersistenceTelemetryEnabled() helper method
- Implemented telemetry span creation in SQLite repository
- Added OpenTelemetry instrumentation to key repository methods:
  - CreateUser: Tracks user creation with error recording
  - GetUserByUsername: Tracks queries with username attribute
- Maintained backward compatibility - telemetry is optional and disabled by default
- Updated all tests to pass config parameter to repository constructor
- Added proper error recording and span attributes for observability

Benefits:
- Performance monitoring of database operations
- Flamegraph generation capability for persistence layer
- Distributed tracing across service boundaries
- Configurable instrumentation for production vs development

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

pkg/config/config.go

@@ -43,11 +43,17 @@ type LoggingConfig struct {
 
 // TelemetryConfig holds OpenTelemetry-related configuration
 type TelemetryConfig struct {
-	Enabled      bool          `mapstructure:"enabled"`
-	OTLPEndpoint string        `mapstructure:"otlp_endpoint"`
-	ServiceName  string        `mapstructure:"service_name"`
-	Insecure     bool          `mapstructure:"insecure"`
-	Sampler      SamplerConfig `mapstructure:"sampler"`
+	Enabled      bool                       `mapstructure:"enabled"`
+	OTLPEndpoint string                     `mapstructure:"otlp_endpoint"`
+	ServiceName  string                     `mapstructure:"service_name"`
+	Insecure     bool                       `mapstructure:"insecure"`
+	Sampler      SamplerConfig              `mapstructure:"sampler"`
+	Persistence  PersistenceTelemetryConfig `mapstructure:"persistence"`
+}
+
+// PersistenceTelemetryConfig holds persistence layer telemetry configuration
+type PersistenceTelemetryConfig struct {
+	Enabled bool `mapstructure:"enabled"`
 }
 
 // APIConfig holds API version configuration
@@ -107,6 +113,7 @@ func LoadConfig() (*Config, error) {
 	v.SetDefault("telemetry.insecure", true)
 	v.SetDefault("telemetry.sampler.type", "parentbased_always_on")
 	v.SetDefault("telemetry.sampler.ratio", 1.0)
+	v.SetDefault("telemetry.persistence.enabled", false)
 
 	// API defaults
 	v.SetDefault("api.v2_enabled", false)
@@ -215,6 +222,11 @@ func (c *Config) GetServiceName() string {
 	return c.Telemetry.ServiceName
 }
 
+// GetPersistenceTelemetryEnabled returns whether persistence layer telemetry is enabled
+func (c *Config) GetPersistenceTelemetryEnabled() bool {
+	return c.Telemetry.Enabled && c.Telemetry.Persistence.Enabled
+}
+
 // GetTelemetryInsecure returns whether to use insecure connection
 func (c *Config) GetTelemetryInsecure() bool {
 	return c.Telemetry.Insecure