Restructure upload validation env and colocate config types.

Move app config shapes under types/config, replace batch upload env vars with
per-image bytes and JSON body padding, and extend IsBase64 for decoded limits.

backend/src/config/validate.ts

@@ -45,12 +45,6 @@ class EnvironmentVariables {
     @IsString()
     CORS_ORIGINS: string;
 
-    @IsNotEmpty()
-    @IsNumber()
-    @Min(1)
-    @Max(50 * 1024 * 1024)
-    MULTER_MAX_FILE_BYTES: number;
-
     @IsNotEmpty()
     @IsIn(BOOLEAN_STRING_VALUES)
     ALTCHA_ENABLED: BooleanStringValue;
@@ -65,6 +59,12 @@ class EnvironmentVariables {
     @Max(1_000_000)
     ALTCHA_MAX_NUMBER: number;
 
+    @IsNotEmpty()
+    @IsNumber()
+    @Min(60_000)
+    @Max(3_600_000)
+    ALTCHA_CHALLENGE_TTL_MS: number;
+
     @IsNotEmpty()
     @IsNumber()
     @Min(1)
@@ -98,14 +98,20 @@ class EnvironmentVariables {
     @IsNotEmpty()
     @IsNumber()
     @Min(1)
-    @Max(100)
-    VALIDATION_UPLOAD_MAX_IMAGES_PER_UPLOAD: number;
+    @Max(10_000_000)
+    VALIDATION_UPLOAD_MAX_VIEWS_MAX: number;
 
     @IsNotEmpty()
     @IsNumber()
     @Min(1)
-    @Max(10_000_000)
-    VALIDATION_UPLOAD_MAX_VIEWS_MAX: number;
+    @Max(50 * 1024 * 1024) // 50MB
+    VALIDATION_UPLOAD_MAX_IMAGE_BYTES: number;
+
+    @IsNotEmpty()
+    @IsNumber()
+    @Min(256 * 1024) // 256KiB
+    @Max(16 * 1024 * 1024) // 16MB
+    VALIDATION_UPLOAD_JSON_BODY_PADDING_BYTES: number;
 }
 
 export const validate = (config: Record<string, unknown>) => {