feat: modelo Log y CRUD en fn_operations

Tipo Log con niveles debug/info/warn/error, source, entity_id y execution_id
opcionales. Migración 003_logs.sql y funciones InsertLog, GetLog, ListLogs
con filtros combinables.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

fn_operations/models.go

@@ -145,6 +145,28 @@ type AssertionResult struct {
 	EvaluatedAt time.Time             `json:"evaluated_at"`
 }
 
+// LogLevel represents the severity of a log entry.
+type LogLevel string
+
+const (
+	LogDebug LogLevel = "debug"
+	LogInfo  LogLevel = "info"
+	LogWarn  LogLevel = "warn"
+	LogError LogLevel = "error"
+)
+
+// Log is a free-form operational event within a project context.
+type Log struct {
+	ID          string         `json:"id"`
+	Level       LogLevel       `json:"level"`
+	Source      string         `json:"source"`       // who: agent, pipeline, reactive_loop, function name...
+	EntityID    string         `json:"entity_id"`    // optional context
+	ExecutionID string         `json:"execution_id"` // optional context
+	Message     string         `json:"message"`
+	Metadata    map[string]any `json:"metadata"`
+	CreatedAt   time.Time      `json:"created_at"`
+}
+
 // TypeSnapshot is an immutable copy of a registry type at point of use.
 type TypeSnapshot struct {
 	ID          string    `json:"id"`