Validate dimensions and fit mode on encrypted URLs (closes #62)

The encrypted /v1/e/ route used the decrypted payload unchecked, so a
token could request an over-limit size or an unknown fit mode; the
generator turned unparseable numbers into 0.

imgcache.ValidateDimension alone holds the MaxDimension bound and is
used by the path parser, by the new ValidateImageRequest (which adds
ValidateFitMode) and by the generator. Both image routes call
ValidateImageRequest, so each answers 400. The generator answers 400
naming the field for a width or height that is not a number or fails
that check, a quality that is not a number from 1 to 100, a ttl that is
not a number from 0 to the largest the expiry calculation can hold, or
an unknown fit. Empty quality is 85; empty ttl never expires. The
form's size inputs stop at 8192.

Model: opus-4-8 (implementation); opus-5-5 (rework)

internal/imgcache/validate_internal_test.go

@@ -0,0 +1,64 @@
+package imgcache
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestValidateImageRequest(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		req     ImageRequest
+		wantErr error
+	}{
+		{
+			name: "within bounds",
+			req:  ImageRequest{Size: Size{Width: 800, Height: 600}, FitMode: FitCover},
+		},
+		{
+			name: "original size and empty fit",
+			req:  ImageRequest{Size: Size{Width: 0, Height: 0}},
+		},
+		{
+			name:    "width over limit",
+			req:     ImageRequest{Size: Size{Width: MaxDimension + 1, Height: 600}},
+			wantErr: ErrDimensionTooLarge,
+		},
+		{
+			name:    "height over limit",
+			req:     ImageRequest{Size: Size{Width: 800, Height: MaxDimension + 1}},
+			wantErr: ErrDimensionTooLarge,
+		},
+		{
+			name:    "negative width",
+			req:     ImageRequest{Size: Size{Width: -1, Height: 600}},
+			wantErr: ErrInvalidSize,
+		},
+		{
+			name:    "invalid fit mode",
+			req:     ImageRequest{Size: Size{Width: 800, Height: 600}, FitMode: "bogus"},
+			wantErr: ErrInvalidFitMode,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			err := ValidateImageRequest(&tt.req)
+			if tt.wantErr == nil {
+				if err != nil {
+					t.Fatalf("ValidateImageRequest() error = %v, want nil", err)
+				}
+
+				return
+			}
+
+			if !errors.Is(err, tt.wantErr) {
+				t.Fatalf("ValidateImageRequest() error = %v, want %v", err, tt.wantErr)
+			}
+		})
+	}
+}