Replace implemented column with CMMI maturity level

Drop the boolean implemented/not-implemented state in favor of a
mandatory CMMI maturity level enum (NONE, INITIAL, MANAGED, DEFINED,
QUANTITATIVELY_MANAGED, OPTIMIZING) stored as a Postgres enum type.

The migration backfills existing rows (NOT_IMPLEMENTED → NONE,
IMPLEMENTED → INITIAL), makes the column NOT NULL, and drops the old
implemented column and its enum type.

- maturityLevel is required on CreateControlInput and non-nullable (!)
  in the GraphQL schema
- CLI displays human-readable CMMI labels instead of raw enum tokens
- SOA table and published document use a single Maturity column in
  place of the old Implemented + Maturity columns
- Remove ControlImplementationState type and all implemented references
  across backend, frontend, CLI, MCP, n8n, and E2E tests

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/coredata/control_maturity_level.go

@@ -32,6 +32,17 @@ const (
 	ControlMaturityLevelOptimizing            ControlMaturityLevel = "OPTIMIZING"
 )
 
+func ControlMaturityLevels() []ControlMaturityLevel {
+	return []ControlMaturityLevel{
+		ControlMaturityLevelNone,
+		ControlMaturityLevelInitial,
+		ControlMaturityLevelManaged,
+		ControlMaturityLevelDefined,
+		ControlMaturityLevelQuantitativelyManaged,
+		ControlMaturityLevelOptimizing,
+	}
+}
+
 func (l ControlMaturityLevel) IsValid() bool {
 	switch l {
 	case ControlMaturityLevelNone,