feat(partnum): implement part number validation (#80)

Implement Generator.Validate() to check part numbers against schemas:
- Split by separator, verify segment count
- Constant: must equal expected value
- Enum: must be in allowed values map
- String: length, case, pattern constraints
- Serial: length + numeric-only check
- Date: length matches expected format output

Add belt-and-suspenders call in HandleCreateItem after Generate().
Add 9 validation tests (all pass alongside 10 existing tests).

Closes #80

internal/api/handlers.go

@@ -621,6 +621,12 @@ func (s *Server) HandleCreateItem(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 
+		if err := s.partgen.Validate(partNumber, schemaName); err != nil {
+			s.logger.Error().Err(err).Str("part_number", partNumber).Msg("generated part number failed validation")
+			writeError(w, http.StatusInternalServerError, "validation_failed", err.Error())
+			return
+		}
+
 		item = &db.Item{
 			PartNumber:      partNumber,
 			ItemType:        itemType,

internal/partnum/generator.go

@@ -178,7 +178,84 @@ func (g *Generator) Validate(partNumber string, schemaName string) error {
 		return fmt.Errorf("unknown schema: %s", schemaName)
 	}
 
-	// TODO: parse part number and validate each segment
-	_ = s
+	parts := strings.Split(partNumber, s.Separator)
+	if len(parts) != len(s.Segments) {
+		return fmt.Errorf("expected %d segments, got %d", len(s.Segments), len(parts))
+	}
+
+	for i, seg := range s.Segments {
+		val := parts[i]
+		if err := g.validateSegment(&seg, val); err != nil {
+			return fmt.Errorf("segment %s: %w", seg.Name, err)
+		}
+	}
+
+	return nil
+}
+
+// validateSegment checks that a single segment value is valid.
+func (g *Generator) validateSegment(seg *schema.Segment, val string) error {
+	switch seg.Type {
+	case "constant":
+		if val != seg.Value {
+			return fmt.Errorf("expected %q, got %q", seg.Value, val)
+		}
+
+	case "enum":
+		if _, ok := seg.Values[val]; !ok {
+			return fmt.Errorf("invalid enum value: %s", val)
+		}
+
+	case "string":
+		if seg.Length > 0 && len(val) != seg.Length {
+			return fmt.Errorf("value must be exactly %d characters", seg.Length)
+		}
+		if seg.MinLength > 0 && len(val) < seg.MinLength {
+			return fmt.Errorf("value must be at least %d characters", seg.MinLength)
+		}
+		if seg.MaxLength > 0 && len(val) > seg.MaxLength {
+			return fmt.Errorf("value must be at most %d characters", seg.MaxLength)
+		}
+		if seg.Case == "upper" && val != strings.ToUpper(val) {
+			return fmt.Errorf("value must be uppercase")
+		}
+		if seg.Case == "lower" && val != strings.ToLower(val) {
+			return fmt.Errorf("value must be lowercase")
+		}
+		if seg.Validation.Pattern != "" {
+			re := regexp.MustCompile(seg.Validation.Pattern)
+			if !re.MatchString(val) {
+				msg := seg.Validation.Message
+				if msg == "" {
+					msg = fmt.Sprintf("value does not match pattern %s", seg.Validation.Pattern)
+				}
+				return fmt.Errorf("%s", msg)
+			}
+		}
+
+	case "serial":
+		if seg.Length > 0 && len(val) != seg.Length {
+			return fmt.Errorf("value must be exactly %d characters", seg.Length)
+		}
+		for _, ch := range val {
+			if ch < '0' || ch > '9' {
+				return fmt.Errorf("serial must be numeric")
+			}
+		}
+
+	case "date":
+		layout := seg.Value
+		if layout == "" {
+			layout = "20060102"
+		}
+		expected := time.Now().UTC().Format(layout)
+		if len(val) != len(expected) {
+			return fmt.Errorf("date segment length mismatch: expected %d, got %d", len(expected), len(val))
+		}
+
+	default:
+		return fmt.Errorf("unknown segment type: %s", seg.Type)
+	}
+
 	return nil
 }

internal/partnum/generator_test.go

@@ -226,6 +226,118 @@ func TestGenerateDateSegmentCustomFormat(t *testing.T) {
 	}
 }
 
+// --- Validation tests ---
+
+func TestValidateBasic(t *testing.T) {
+	s := testSchema()
+	gen := NewGenerator(map[string]*schema.Schema{"test": s}, &mockSeqStore{})
+
+	if err := gen.Validate("F01-0001", "test"); err != nil {
+		t.Fatalf("expected valid, got error: %v", err)
+	}
+}
+
+func TestValidateWrongSegmentCount(t *testing.T) {
+	s := testSchema()
+	gen := NewGenerator(map[string]*schema.Schema{"test": s}, &mockSeqStore{})
+
+	if err := gen.Validate("F01-0001-EXTRA", "test"); err == nil {
+		t.Fatal("expected error for wrong segment count")
+	}
+}
+
+func TestValidateInvalidEnum(t *testing.T) {
+	s := testSchema()
+	gen := NewGenerator(map[string]*schema.Schema{"test": s}, &mockSeqStore{})
+
+	if err := gen.Validate("ZZZ-0001", "test"); err == nil {
+		t.Fatal("expected error for invalid enum value")
+	}
+}
+
+func TestValidateNonNumericSerial(t *testing.T) {
+	s := testSchema()
+	gen := NewGenerator(map[string]*schema.Schema{"test": s}, &mockSeqStore{})
+
+	if err := gen.Validate("F01-ABCD", "test"); err == nil {
+		t.Fatal("expected error for non-numeric serial")
+	}
+}
+
+func TestValidateSerialWrongLength(t *testing.T) {
+	s := testSchema()
+	gen := NewGenerator(map[string]*schema.Schema{"test": s}, &mockSeqStore{})
+
+	if err := gen.Validate("F01-01", "test"); err == nil {
+		t.Fatal("expected error for wrong serial length")
+	}
+}
+
+func TestValidateConstantSegment(t *testing.T) {
+	s := &schema.Schema{
+		Name:      "const-val",
+		Version:   1,
+		Separator: "-",
+		Segments: []schema.Segment{
+			{Name: "prefix", Type: "constant", Value: "KS"},
+			{Name: "serial", Type: "serial", Length: 4},
+		},
+	}
+	gen := NewGenerator(map[string]*schema.Schema{"const-val": s}, &mockSeqStore{})
+
+	if err := gen.Validate("KS-0001", "const-val"); err != nil {
+		t.Fatalf("expected valid, got error: %v", err)
+	}
+	if err := gen.Validate("XX-0001", "const-val"); err == nil {
+		t.Fatal("expected error for wrong constant value")
+	}
+}
+
+func TestValidateUnknownSchema(t *testing.T) {
+	gen := NewGenerator(map[string]*schema.Schema{}, &mockSeqStore{})
+
+	if err := gen.Validate("F01-0001", "nonexistent"); err == nil {
+		t.Fatal("expected error for unknown schema")
+	}
+}
+
+func TestValidateDateSegment(t *testing.T) {
+	s := &schema.Schema{
+		Name:      "date-val",
+		Version:   1,
+		Separator: "-",
+		Segments: []schema.Segment{
+			{Name: "date", Type: "date"},
+			{Name: "serial", Type: "serial", Length: 3},
+		},
+	}
+	gen := NewGenerator(map[string]*schema.Schema{"date-val": s}, &mockSeqStore{})
+
+	today := time.Now().UTC().Format("20060102")
+	if err := gen.Validate(today+"-001", "date-val"); err != nil {
+		t.Fatalf("expected valid, got error: %v", err)
+	}
+	if err := gen.Validate("20-001", "date-val"); err == nil {
+		t.Fatal("expected error for wrong date length")
+	}
+}
+
+func TestValidateGeneratedOutput(t *testing.T) {
+	s := testSchema()
+	gen := NewGenerator(map[string]*schema.Schema{"test": s}, &mockSeqStore{})
+
+	pn, err := gen.Generate(context.Background(), Input{
+		SchemaName: "test",
+		Values:     map[string]string{"category": "F01"},
+	})
+	if err != nil {
+		t.Fatalf("Generate error: %v", err)
+	}
+	if err := gen.Validate(pn, "test"); err != nil {
+		t.Fatalf("generated part number %q failed validation: %v", pn, err)
+	}
+}
+
 func TestGenerateDateSegmentYearOnly(t *testing.T) {
 	s := &schema.Schema{
 		Name:      "date-year",